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Recent theoretical work shows that "electron exhausts", elongated regions of depleted electron density shown by PIC codes to be generated in collisionless reconnection, can in principle become sources of X-mode Auroral Kilometric Radiation generated by the electron cyclotron maser instability (ECMI). In this paper it is suggested that under certain conditions such X-mode AKR can possibly be transported down along the auroral magnetic flux tubes to low altitudes, in extreme cases even penetrating below the ionospheric F- and E-regions to reach ground level. The characteristic features of low-altitude AKR produced and transported by this mechanism would be bandwidths of order $\sim$100 kHz and occurrence in quasi-periodic bursts lasting $< 1$ s when passing at full Alfv\'en speed, $>1$ s when retarded. Most published observations of low-altitude AKR have insufficient time resolution to detect these features, although a few are suggestive of them. The few observations with marginally sufficient time resolution are so far neither consistent nor inconsistent with these predictions, suggesting that occasions when electron exhausts transport AKR all the way to low altitudes may be rare. However, vigorous experimental effort to test these theoretical predictions is warranted because of its physical and astrophysical significance. They would also provide an additional method to remotely detect and investigate reconnection physics., Comment: 9 pages, 3 figures, conference contribution

Recent theoretical work shows that "electron exhausts", elongated regions of depleted electron density shown by PIC codes to be generated in collisionless reconnection, can in principle become sources of X-mode Auroral Kilometric Radiation generated by the electron cyclotron maser instability (ECMI). In this paper it is suggested that under certain conditions such X-mode AKR can possibly be transported down along the auroral magnetic flux tubes to low altitudes, in extreme cases even penetrating below the ionospheric F- and E-regions to reach ground level. The characteristic features of low-altitude AKR produced and transported by this mechanism would be bandwidths of order $\sim$100 kHz and occurrence in quasi-periodic bursts lasting $< 1$ s when passing at full Alfv\'en speed, $>1$ s when retarded. Most published observations of low-altitude AKR have insufficient time resolution to detect these features, although a few are suggestive of them. The few observations with marginally sufficient time resolution are so far neither consistent nor inconsistent with these predictions, suggesting that occasions when electron exhausts transport AKR all the way to low altitudes may be rare. However, vigorous experimental effort to test these theoretical predictions is warranted because of its physical and astrophysical significance. They would also provide an additional method to remotely detect and investigate reconnection physics., Comment: 9 pages, 3 figures, conference contribution

An attractive mechanism for radiation transport by electron holes from the magnetospheric auroral cavity source region down to the ionosphere and possibly even down to the atmosphere is examined. Because of the excitation and propagation properties of the X mode, this mechanism turns out to be highly improbable for the usual resonant excitation of radiation of frequency just below the local electron cyclotron frequency, $\omega \lesssim \omega_{ce}$. It could work only, if the auroral ionosphere would be locally perforated being of sufficiently low density for allowing electron holes riding down to low altitudes on the auroral electron beam. If resonant excitation of higher electron cyclotron harmonics, $\omega \sim \ell \omega_{ce}, ~\ell= 2,3,...$, becomes possible, a still unexplored mechanism, then radiation excited inside the hole could be transported to lower altitudes than generated. If this happens, radiation would provide another mechanism of coupling between the magnetospheric plasma and the atmosphere. Keywords: Electron cyclotron maser, electron holes, auroral acceleration, auroral radiation fine structure, auroral kilometric radiation, Jupiter radio emission, Planetary radio emission, Comment: 4 pages, 1 figure, submitted to Ann. Geophys

An attractive mechanism for radiation transport by electron holes from the magnetospheric auroral cavity source region down to the ionosphere and possibly even down to the atmosphere is examined. Because of the excitation and propagation properties of the X mode, this mechanism turns out to be highly improbable for the usual resonant excitation of radiation of frequency just below the local electron cyclotron frequency, $\omega \lesssim \omega_{ce}$. It could work only, if the auroral ionosphere would be locally perforated being of sufficiently low density for allowing electron holes riding down to low altitudes on the auroral electron beam. If resonant excitation of higher electron cyclotron harmonics, $\omega \sim \ell \omega_{ce}, ~\ell= 2,3,...$, becomes possible, a still unexplored mechanism, then radiation excited inside the hole could be transported to lower altitudes than generated. If this happens, radiation would provide another mechanism of coupling between the magnetospheric plasma and the atmosphere. Keywords: Electron cyclotron maser, electron holes, auroral acceleration, auroral radiation fine structure, auroral kilometric radiation, Jupiter radio emission, Planetary radio emission, Comment: 4 pages, 1 figure, submitted to Ann. Geophys

The Earth’s aurorae radiate away up to 1% of their energy in the form of radio waves, called Auroral Kilometric Radiation (AKR). The mechanism responsible for the emission, the electron cyclotron maser (ECM), produces similar emissions at other planets, in the solar atmosphere, and in astrophysical systems. AKR was not unambiguously identified until the 1970’s because its detection requires a suitably instrumented satellite. The ECM theory predicts radiation beamed outward that cannot penetrate the increasing magnetic field and electron density near the Earth. Nevertheless, there have been observations over the years of AKR-like radio signals detected by ground-based, rocket-borne, and low-earth orbiting satellite-borne instruments, raising the question of whether a mechanism exists by which AKR can penetrate to low altitudes. Here we show the first unambiguous evidence that AKR indeed penetrates to low altitudes on occasions. We identified three examples of AKR-like emissions detected with a ground-based radio receiver at South Pole Station, Antarctica, during a 9-day interval in July, 2004, when the Geotail satellite, monitoring AKR, had a field of view including the auroral field lines above the station. The AKR-like emissions detected at ground-level have the same frequenc–time structure as simultaneous AKR emissions detected on Geotail 115,000-190,000 km away from the Earth. Slight differences in the frequency extent of the emissions at the two locations can be explained by, for example, plasmaspheric screening of the emissions detected by Geotail. These observations represent the first coincident detections of AKR in space and on the ground. They require the existence of an as-yet unidentified mechanism to produce the ground-level emissions which are not predicted by ECM theory, they suggest that previous AKR-like emissions observed at low altitudes may indeed be AKR, and they require revision of the widely-held view that AKR is only detectable from space.

Auroral Medium Frequency Burst (MFB) is the least understood of three types of terrestrial auroral emissions detectable at ground level. MFB consists of broadband (500-2000 kHz) left-polarized impulsive emissions typically occurring for a few minutes at the onset of polar substorms, one of the most energetic phenomena in the terrestrial magnetosphere. Recent observations of the source location and fine structure of MFB provide the best opportunity yet to test theoretical models of the generation mechanism. Proposed mechanisms include mode conversion of Langmuir or electron cyclotron sound waves excited via resonant interactions with auroral electrons; the former has been shown under certain conditions to predict the frequency-time characteristics of MFB fine structure.

The Earth’s aurorae radiate away up to 1% of their energy in the form of radio waves, called Auroral Kilometric Radiation (AKR). The mechanism responsible for the emission, the electron cyclotron maser (ECM), produces similar emissions at other planets, in the solar atmosphere, and in astrophysical systems. AKR was not unambiguously identified until the 1970’s because its detection requires a suitably instrumented satellite. The ECM theory predicts radiation beamed outward that cannot penetrate the increasing magnetic field and electron density near the Earth. Nevertheless, there have been observations over the years of AKR-like radio signals detected by ground-based, rocket-borne, and low-earth orbiting satellite-borne instruments, raising the question of whether a mechanism exists by which AKR can penetrate to low altitudes. Here we show the first unambiguous evidence that AKR indeed penetrates to low altitudes on occasions. We identified three examples of AKR-like emissions detected with a ground-based radio receiver at South Pole Station, Antarctica, during a 9-day interval in July, 2004, when the Geotail satellite, monitoring AKR, had a field of view including the auroral field lines above the station. The AKR-like emissions detected at ground-level have the same frequenc–time structure as simultaneous AKR emissions detected on Geotail 115,000-190,000 km away from the Earth. Slight differences in the frequency extent of the emissions at the two locations can be explained by, for example, plasmaspheric screening of the emissions detected by Geotail. These observations represent the first coincident detections of AKR in space and on the ground. They require the existence of an as-yet unidentified mechanism to produce the ground-level emissions which are not predicted by ECM theory, they suggest that previous AKR-like emissions observed at low altitudes may indeed be AKR, and they require revision of the widely-held view that AKR is only detectable from space.

Auroral Medium Frequency Burst (MFB) is the least understood of three types of terrestrial auroral emissions detectable at ground level. MFB consists of broadband (500-2000 kHz) left-polarized impulsive emissions typically occurring for a few minutes at the onset of polar substorms, one of the most energetic phenomena in the terrestrial magnetosphere. Recent observations of the source location and fine structure of MFB provide the best opportunity yet to test theoretical models of the generation mechanism. Proposed mechanisms include mode conversion of Langmuir or electron cyclotron sound waves excited via resonant interactions with auroral electrons; the former has been shown under certain conditions to predict the frequency-time characteristics of MFB fine structure.

Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.

Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.