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The Active Plasma Experiment uses intensive electron beam emission for the study of dynamic processes in the magnetosphere and upper ionosphere. The beam energy and current are as high as 8 keV and 100 mA and the pitch angle of the emission varies in the range from 50° to 80°. The basic cycle of electron injection is formed by current pulses of different duration, intensity, and frequency. The spacecraft potential is balanced by a low-energy xenon plasma generator during the electron beam injection. The spacecraft potential is measured by the floating probe, and the response of the environment is studied by the charged particle spectrometer working in the energy range from 0.05 to 25 keV. During the neutral or ionized Xe release without the electron gun operation the spacecraft potential remains nearly unchanged and the observed energy spectra of charged particles do not exhibit the presence of any acceleration process. The spacecraft potential during electron beam emission does not exceed 50 V if the Xe plasma or the neutral gas was released together with the electrons. The electron gun firing creates a disturbance which produces a broad spectrum of energetic electrons extending up to 1.5 keV. The acceleration process can be explained by the introduction of the electric field with intensity of about 100 V/m. This intensity is in agreement with the observed E × B drift velocity. The spatial extent of the disturbance is established to be tens of meters.

Plasma density and plasma wave measurements aboard the "Vega-2" spacecraft discovered two plasma density enhancements in the inner coma of comet Halley that are accompanied by bursts of plasma waves in the lower-hybrid frequency range. These events are explained by the critical ionization velocity (CIV) effects of Alfven. The persistent bursty plasma wave activity and the intermittent plasma density growth toward the cometary nucleus indicate that the CIV effect contributes to the ionization of the cometary gas.

Energetic particle observations made by the Irish SLED instrument on the Phobos 2 spacecraft in 1989 have revealed the presence, within the overall energy range 3.2 MeV, of variously located energetic particle populations in the close Marian environment. The signatures of characteristic boundaries have also been recorded for the first time in energetic particles in the distant Martian magnetotail. The new SLED-II instrument on the Mars-94 Mission is designed to study in detail, with 4π measurement capability, these and other energetic particle phenomena at Mars, while operating, over an extended period, at low altitudes above the planet.

All the available nightside electron data obtained during circular orbits at Mars from the Phobos-2 Hyperbolic Retarded Potential Analyzer (HARP) instrument have been examined in detail and are summarized in this paper. An electron flux component with energies exceeding that of the unperturbed solar wind was observed inside the magnetosheath, indicating the presence of acceleration mechanism(s). The character of the electron fluxes measured in the magnetotail cannot be classified in any simple manner, however, there is a correlation between the electron fluxes measured well inside this region and the unperturbed solar wind ram pressure.

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three-dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.

During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured by Ap was found to correlate with the square of solar wind speed V, and activity measured by Dst was found to correlate with the product of V and the southward component of the interplanetary magnetic field, B[sub s]. Both of these correlations break down during cycle 21. In the case of Ap, the much stronger variation of B[sub s] in cycle 21 compared to cycle 20 makes clear that the B[sub s] contribution to activity is important on yearly as well as shorter time scales. The product B[sub s]V[sup 2] gives an excellent correlation with Ap over both cycles. In the case of Dst, the stronger variation of B[sub s] in cycle 21 causes a stronger variation in B[sub s]V, which is not reflected in Dst, perhaps because Dst also depends upon solar wind dynamic pressure in a nonlinear way. 12 refs., 4 figs.

Measurements of proton and heavy ion spectra by the TAUS spectrometer onboard the Phobos-2 orbiter provided the first in-situ experimental data on the problem of martian atmosphere dissipation. They are (i) the newly revealed escape of planetary heavy ions through the plasma sheet of the martian magnetotail, and (ii) the deceleration of solar wind protons upstream of the planetary bow shock possibly due to the presence of the hot oxygen corona of Mars.

A preliminary overview of particle records obtained by the SLED instrument on Phobos 2, February–March, 1989 during Mars encounter, is presented. Data obtained while in close elliptical orbit around the planet (pericenter < 900 km), in both spin and three axis stabilised mode, display evidence of energy related particle shadowing by the body of Mars. This effect was also observed, under favourable conditions, in certain circular orbits (altitude 6330 km above the planet). Flux enhancements, inside the magnetopause, in the approximate range 30–350 keV, recorded in the same general location at < 900 km above Mars over an 8 day period during three consecutive elliptical orbits, are described. Possible explanations of these enhancements include the presence of quasi-trapped radiation at the planet and the detection of the propagation of accelerated particles along the boundary of the magnetopause from the day to the night side of Mars. Large anisotropic ion flux increases (1–1.5 orders of magnitude) in the approximate range 30–200 keV recorded in front of the bow shock (inbound and outbound) during certain circular orbits, provide evidence that the spacecraft traversed strongly anisotopic jets of energetic particles. These are suggested to have constituted O+ ions. The pickup process would have been sufficient to accelerate such ions to their observed energies in the prevailing solar wind conditions. Alternatively, they might have comprised particles that had leaked from inside the magnetopause, perhaps undergoing shock drift acceleration in the process. Significant flux enhancements were also sometimes identified in the magnetotail (approximate energy range 30–50 keV). These are suggested to represent the signatures of O+ beams, impelled by acceleration processes similar to those associated with terrestrial ion beams.

Even though magnetic field and plasma in-situ measurements near Mars from the 1989 PHOBOS-2 project and from earlier missions are available, the existence of an Martian intrinsic magnetic field is still controversial. In this study we analyze data of the PHOBOS-2 magnetic field experiments MAGMA and FGMM and use the upstream solar wind parameters of the TAUS and ASPERA experiments. Different methods are used to investigate the influence of the interplanetary magnetic field (IMF) and of a possible weak intrinsic field on the solar wind interaction with Mars : The compressibility of plasma boundaries, the correlation between upstream IMF and tail properties and between magnetic field structures and planetary rotation. The study shows that the magnetic field in the tail is strongly correlated with the upstream IMF suggesting that the Martian magnetotail is induced, at least to a large extent. Compressibility studies reveal a weak dependence of the plasma boundaries on the solar wind dynamic pressure but the bow shock location appears to be not affected by the Martian longitude within the accuracy of our measurements. We conclude that an intrinsic planetary field, if it exists, does not play a major role in the interaction between the solar wind and Mars.

The electron distribution functions measured in the neighborhood of Mars by means of the Hyperbolic Retarding Potential Analyzer (HARP) carried aboard the Phobos 2 spacecraft are presented. The measurements were carried out over an energy/charge ( E / q ) from ∼0.3 eV to ∼800 eV in eight independent angular sectors ∼ 20° × 10° covering the FOV ∼ 180° in the X - Z plane in the antisolar directions. The total intensity and energy distribution function of electrons downstream of the bow shock clearly differ from those in the undisturbed solar wind. The electron fluxes are significantly increased and the energy distribution of electrons in the magnetosheath was found to be characterized by the double-peaked structure. The high energy fluxes often exceed the flux values for the low energy peak.

The Low Energy Telescope (LET) experiment carried on board the Phobos spacecraft measured the flux, spectra and elemental composition of nuclei from hydrogen up to iron, in the energy range ∼ 1–75 MeV nucleon −1 . Isotope separation for helium was also achieved. We present the results of a study of solar energetic particle (SEP) composition using LET data acquired during the period July 1988 to March 1989. The set of particle events selected for study comprises six large solar flare events, two 3 He-rich events and two energetic storm particle increases associated with interplanetary shocks. Three of the six large flare events occurred during a period of unusually high solar activity in March 1989. In two of these events, large Fe/O ratios were measured (0.44 ± 0.05 and 0.95 ± 0.24). The Fe/O ratios determined for the complete set of large flare events show an inverse correlation with the spectral index of oxygen, suggesting that the acceleration mechanism that produces events showing enhanced heavy-ion abundances is different from that responsible for lower Fe/O ratios. Our results in the case of the 3 He-rich events are in agreement with earlier work, showing an enrichment in heavy ions relative to the average SEP composition. The abundances measured for the shock-associated increases are consistent with the acceleration of the ambient population of solar flare particles.

The charged particle detector SLED on the Phobos-2 spacecraft has recorded, during a number of circular orbits about Mars, significant fluxes of ions with energies up to 200 keV in close spatial association with the Martian bow shock. The observed characteristics of these enhancements suggest that different shock acceleration mechanisms were operative in producing individual events

Twin telescope particle detector systems SLED-1 and SLED-2, with the capability of monitoring electron and ion fluxes within an energy range spanning approximately 30 keV to a few megaelectron volts, were individually launched on the two spacecraft ( Phobos -2 and Phobos -1, respectively) of the Soviet Phobos Mission to Mars and its moons in July 1988. A short description of the SLED instrument and a preliminary account of representative solar-related particle enhancements recorded by SLED-1 and SLED-2 during the Cruise Phase, and by SLED-1 in the near Martian environment (within the interval 25 July 1988-26 March 1989) are presented. These observations were made while the interplanetary medium was in the course of changing over from solar minimum- to solar maximum-dominated conditions and examples are presented of events associated with each of these phenomenological states.

The HARP differential electrostatic analyzer measured thermal and supra thermal electron and ion fluxes and distributions in the Mars environment. High bit-rate data obtained around the pericenter passage on elliptical orbits early in February 1989 will be highlighted, and correlation with results of other experiments will be discussed. A similarity between the HARP total count rate variations and the spacecraft potential measurements of the Plasma-Wave System experiment is established. The variation of electron energy spectra is discussed in various regions of the solar wind interaction with Mars. Tentative results on electron and ion anisotropy are presented, making use of regular variations of the ratio of count rates of two sensor heads looking in perpendicular directions. Magnetic field data of the MAGMA magnetometer are used for comparison.

A low energy particle detector system (SLED) is described which was designed to measure the flux densities of electrons and ions in the energy range from ≈30 keV to a few MeV in (a) the varying solar aspect angles and temperatures pertaining during the Cruise Phase of the Phobos Mission and (b) in the low temperature environment (reaching −25° C) pertaining during Mars Encounter. Representative data illustrating the excellent functioning of SLED during both phases of the mission are presented.

The HARP instrument is a hyperbolic electrostatic analyzer working in the retarding potential mode. It is the lowest-energy member of the ESTER particle detector family. The energy range extends from 0.25 eV to 850 eV for both electrons and ions. The eight viewing sectors are arranged in a fan-shaped geometry in the antisolar hemisphere. They are simultaneously sampled while energy is stepped over a maximum number of 75 logarithmically spaced channels. The instrument is intended for thermal and superthermal solar-wind electron observations during the cruise phase and — more importantly — for electron and ion observations in the Mars environment.

The only source of experimental data on martian magnetosphere before 1989 was the results of plasma and magnetic field measurements from Mars orbiters Mars 2, 3 and 5. The features of martian magnetosphere similar to proper features of magnetospheres of Earth and Venus were revealed; some evidences in favour of the existence of martian intrinsic magnetic field are mentioned.

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (about 10 exp 8/sq cm per sec) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the planet.

The measurements of proton spectra obtained by the TAUS spectrometer on board the Phobos 2 spacecraft in elliptical orbits near Mars are presented. A strong deceleration of the solar wind upstream of the Martian bow shock was revealed. It can be caused by the mass loading of the plasma flow by ions originating from the hot oxygen/hydrogen corona of Mars and/or by protons specularly reflected from the bow shock. In the first case the deceleration of the solar wind by about 100 km/s implies that the hot oxygen corona of Mars could be several times denser than it was anticipated to be (at least during the observation period that was close to solar cycle maximum). Furthermore, the loss of planetary oxygen through the corona appears to be the main process of oxygen loss from Mars. The upper limit of loss rate for such a process is determined to be 1026 oxygen atoms or 2.5 kg of oxygen per second.

The widely used concept of the plasmapause as the last closed electric field equipotential in the equatorial plane of the magnetosphere is oversimplified. The field aligned plasma motions are of substantial importance in the plasmapause formation and should be taken into account. Distributions of the main plasma parameters measured from the Prognoz-5 satellite are presented. The diurnal variations of the plasmapause height and the plasmasphere thermal properties are discussed.

The effect of solar wind loading by this phenomenon developed by Sagdeev et al. cometary ions on the position and structure of (1986), the characteristic frequency of the the comet Halley bow shock is discussed on the excited waves is of the order o2 the heavy io basis of simultaneous measurements of plasma, gyrofrequency m . = eB/m.c  10 Hz (for H20 c1 magnetic field and plasma waves aboard the "Vega- ions) in agreement withthe observations near 1" and "Vega-2" spacecraft. Data from the inbound comets Giacobini-Zinner and Halley (Tsurutani and crossings of the bow shock show that both Smith, 1986; Riedler et al., 1986). quasiperpendiuclar ("Vega-l") and quasiparallel The dependence of the wave energy density W = ("Vega-2") shocks were observed. The thickness of ZIB. I 2 on the distance r from the cometary these shocks is greater than that of the Earth's nucleus is described by the following equation bow shock at least by the ratio of the masses of cometary ions and protons. The bow shock position dW = AQ mi VA is reasonably well described by the kinetic model dx 2 V z of solar wind loading by cometary ions. r g The process of solar wind mass-loading by cometary ions implanted in the supersonic solar wind flow due to the photoionization of gas evaporated from the cometary nucleus is well m W 2 s ci

Plasma observations near comet Halley indicate that around 1.6x105km from the nucleus a newly discovered sharp boundary (cometopause) separates the solar wind controlled external and the heavy cometary ion dominated internal regions. Such a discontinuity was previously not predicted by theoretical models. Inside the cometopause (in the cometary plasma region) the protons and heavy ions move with different speeds: the heavy ion velocity is less than a few km/s throughout this region, while the protons decelerate from several tens of kn.s (observed near the cometopause) to a few km/s (near 1.5x10'km).

The Giotto, Vega-1 and Vega-2 spacecraft flew through the environment of comet Halley at a relatively close range with velocities of the order of 70–80 km/s. The fore sections of their surface were bombarded by neutral molecules and dust grains which caused the emission of secondary electrons and sputtered ions. This paper makes use of the secondary electron current measurements performed on Vega-1 to infer some characteristic features of the cometary atmosphere. The total gas production rate is estimated to be of the order of 10 30 molecules/s and is found to vary with time; the presence of a major jet is also detected at closest approach.

We present the first in situ observations and a description of the large-scale behaviour of comet Halley's plasma environment. The PLASMAG-1 experiment, carried aboard the spacecraft Vega 1 and Vega 2, had the following aims: (1) to study the change of plasma parameters and distributions as a function of distance from the nucleus; (2) to investigate the existence and structure of the cometary bow shock; (3) to determine the change in chemical composition of the heavily mass-loaded plasma as the spacecraft approached the comet; and (4) to measure the neutral gas distribution along the spacecraft trajectory. We observe a discontinuity (the ‘cometopause’) between the solar-wind-controlled cometosheath and heavy-ion mantle and the magnetized cometary plasma region. From the measured neutral gas density distribution we estimate a total gas production rate of 1.3×1030 molecules s−1.

The brief review of the properties of dayside and nightside ionosphere of Venus is given, and some possible origins of ionization are discussed. There are arguments in favour of the impact ionization by electrons from magnetotail as the source of main ionization peak of the nightside ionosphere. The discrepancies in the results obtained by the different in-situ experimental techniques, as well as the discrepancies between the in-situ experimental results and radio occultation results, do not allow the Venus ionosphere to be described quantitatively (although the qualitative description seems to be possible). After Venera-9, −10 and Pioneer-Venus experiments, a great number of publications followed which dealt with experimental data, hypotheses and theoretical models. They described some properties of Venus' ionosphere as well as its origin. Measurements of the same properties made by different methods often gave inconsistent results. Models based on various preconditions had both common and different features. Finally hypotheses of ionization sources, especially in the night ionosphere, are still under discussion. It is of interest to analyze the available publications about the properties and origin of Venus' ionosphere emphasizing the inconsistent and debatable results in order to argue for the necessity of statistically processing the available information, its averaging and tabulation for VIRA.

Based on the ion plasma and magnetic field observations of Vega−1 near its closest approach to comet Halley a self-consistent scenario is developed according to which the observed magnetic field topology, the observed burst of ions at energies 200–600 eV, and the observed directional dependence of the flow of these ions leads to the conclusion that these burst-particles are cometary ions which have been accelerated by the process of merging of magnetic field lines of opposite polarity.

Ion temperature and total ion concentration measured on 25th October 1977 during the flight of the geophysical rocket “Vertical-6” are analyzed. The solar EUV fluxes determined in five wave-length bands with a photoelectron analyzer are also given. The observed anomalous variation of ion temperature between 700 and 900 km and the measured ion concentration can be explained, if the charge exchange reactions H + ⇌ O + and diffusion are taken into account.

Based on the ion, electron and neutral gas observations, performed by five of the six sensors comprising the PLASMAG-1 experiment on board VEGA-1 and -2, the following results are discussed: (1) the existence of the bow shock and its location at ≀ 1.1×106 km for VEGA-1 inbound; (2) the existence of a cometopause and its location at ≀ 1.6×105 km for VEGA-2 inbound; (3) the plasma dynamical processes occurring inside the cometosheath; (4) the phenomena taking place within the cometary plasma region including mass-spectroscopy of cometary ions at distances ≀ 1.5×104 km; (5) the existence of keV electrons near closest approach to the nucleus; and (6) the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of ≀ 2×106 km and an overall production rate of ≀ 1.3×1030 molecules s−1 for VEGA-1 inbound. The results are also discussed in the context of the other, both remote and in-situ, observations, performed on board the VEGA- and GIOTTO-spacecraft.

(1) Investigation by space probes have made it possible to obtain the altitude distributions of the charged particle densities on the sunlit and dark sides of Venus and in the daytime ionosphere of Mars. (2) The planets' ionospheres appear to be less dense and extended than the earth's and the regions of maximum electron density also lie much lower than in the earth's ionosphere. (3) The outer boundary of the thermal plasma in the ionosphere of Venus lies much lower than in the earth's ionosphere (at altitudes of about 500–600 km from the surface on the planet's day-side while in the earth's ionosphere it lies at a height of about 30 000 km), which is mainly due to the fact that Venus has no intrinsic magnetic field. (4) The earth's magnetic field is responsible for some essential peculiarities of the structure of the earth's ionosphere, which are absent in the ionospheres of Mars and Venus. (5) A final choice between models of the types of the F2, F1 or E layers for describing the Martian and Venusian ionospheres by analogy with the earth's ionosphere is still premature since there are no unambiguous data on the composition and temperatures of the planets' neutral upper atmospheres.

A preliminary analysis is presented of the first results on plasma electrons near Mars obtained by retarding potential analyzers onboard the Mars-2 and Mars-3 orbiters. Two zones of significantly increased electron density or temperature were uncovered; one near Mars, which is interpreted as the solar wind shock front in interaction with Mars, and the other >10 5 km from Mars.

T paper brieffy describes the system of radio instruments employed in the study of the distribution of electron concentration with height in the ionosphere, employing the vertically launched geophysical rockets of the Acadenry of Sciences USSR ( l^ ) . 1 The system comprises: a) A system of radio transmitters and transmitting antennas installed on the rocket, b) ground receiving antennas, c) receivingphase-measuring equipment, and d) recording and auxiliary equipment. The work was based on the dispersion interferometer principle proposed by Mandel'shtam and Papaleski in 1937 (3). The method of measurement, basis for selecting the radio frequencies and the measurement results are set out in Ref. 4. It will be said here only that the measurements consisted in the determination of the change in phase difference of two coherent oscillations emitted from the rocket when received on the ground as the result of radio wave dispersion in the ionosphere. For this purpose, from 1954 to 1958 radio waves were used with frequencies fx = 144 Mc per sec and f2 = 48 Mc per sec; in addition to these frequencies, from 1958 a third frequency /3 = 24 Mc per sec was used (with additional recording of the phase difference on the frequencies /i and fz). The phase difference of two coherent signals with different frequencies fx = pf2 and /2, where p > 1 is defined as the quantity Ap = (pi — p(p2, i.e., the phase difference referred to the higher frequency. In order to calculate the mean electron density ne over the altitude range Ah corresponding to the measured phase difference variation A , use was made of equation quency, ne can readily be determined over the altitude range Ah, corresponding to rotation through the angle d

Data from the second Soviet space rocket are analyzed. Ion concentrations are evaluated from collector-current data frora traps; knowledge of the container electric potential is important. Current variations in a "+15 v" trap are analyzed. Space distributions of radiation belts surrounding the earth are proposed. It is suggested that the soft-electron concentration has a minimum in the radtation-beit zones. Plasma ion concentrations are evaluated from indications of a zero-potential trap. Assuming the plasma ions to be protons, thermal velocities are near the container velocity. Data indicate that the investigated plasma is not interplanetary ionized gas. It is concluded that the zodiacal-light polarized componert is due to sunlight scattering by dust Particles and not free electrons. (L.H.N.)

Preliminary results of observations of plasma electrons with energies up to 400 eV, measured on Mars 2 and Mars 3, were previously presented by Gringanz et al. (1973). A summary of the results of further reduction of the data obtained and an attempt at their interpretation is given. (WDM) Two shocklike structures (i.e., abrupt steplike increases in number density, bulk speed, and proton temperature) were found in the data from Martiner 5 at 0.98 AU on June 26 and at 0.85 AU on August 29, 1967. The thickness of these structures' determined from the magnetic field data was more than 1,000 proton Larmor radii. They are also observed by Explorer 33, 34, and 35 at 1 Au, where the thickness were much smaller than in the Mariner 5 data. It is suggested that these two structures were nonlinear magnetoacoustic waves that were in the processes of steepening. It is further suggested that shocks would be formed just beyond 1 AU. These structures were not associated with corotating streams, but they could be associated with impressive solar events in which a flare was followed by type 2 and type 4 radio emission and a rare chromospheric wave. (auth)

THE twin-telescope particle-detector system, SLED, aboard Phobos 2 recorded flux enhancements in the range 30–350 keV in the same general location in the close environment of Mars, over eight days at ∼900 km altitude in three successive elliptical orbits. Here we present possible interpretations of these observations. Energy-related particle shadowing by the body of Mars was also detected, and the data indicate that this effect occurred in

The space probes which flew through the environment of Comet Halley have been bombarded by a flow of molecules and dust particles which impacted their surface with a relative velocity of the order of 70-80 km s−1. The emission of secondary electrons and sputtered ions caused by these impacts was a potential source of interference for the experiments which analyzed the gas and plasma environment of the comet. The impact plasma detector is a simple device which measured the saturation current of the secondary electrons emitted from a gold target mounted on Vega-1 and Vega-2. The effects of the gas and dust impacts can be easily separated; the total gas production rate of the nucleus is estimated to be of the order of 1030 molecules/s at 0.8 AU.

Energetic cometary ion fluxes measured between 2.5106 km and 106 km from the cometary nucleus along the inbound trajectory of s/c VEGA-1 are used to derive the temperatures of the ion distributions in the solar wind frame. The increase of the temperature is modelled by the temperature change derived from a Fokker-Planck type equation with a source term and a stochastic acceleration term. The temperature increase predicted by theory is about 3 keV, higher than the observed one (≃ 1.4 keV). The difference may be due to the approximations applied. The second order Fermi mechanism is thus capable of producing the temperature increase observed.