Southern Auroral Zone Research Articles

The Effects of a Small Geomagnetic Storm on Earth's Thermosphere and Ionosphere: ICON Observations of the 25 January 2021 Disturbance

AbstractIsolated geomagnetic storms during solar minimum offer the opportunity to observe the effects of solar wind on Earth's atmosphere without interference from other background changes in the ionosphere‐thermosphere system. Here, we observe the effects of a minor storm on the ionosphere at middle and low latitudes, where disturbances in thermospheric winds and composition can produce storm‐time changes in ionospheric densities. Using data from the NASA Ionospheric Connection Explorer (ICON), we investigate the competing effects of composition changes and neutral wind disturbances in modifying the ionosphere during the 25 January 2021 storm. Large enhancements in plasma densities are seen across the dayside late on 25 January, associated with large changes in meridional winds and significant high‐latitude electric current enhancements as measured by the auroral electrojet index. At the same time, we find evidence of rapid transport of heated air from the southern auroral zone to the equator in the East‐Asian sector with the appearance of deeply reduced O/N2 in the morning sector and storm‐time northward winds prior to its appearance. We observe significant, rapid erosion of the high plasma densities in the sector where thermospheric disturbance appears over 5 hr of observation. The noon‐time plasma density enhancements are short‐lived and we observe no westward perturbation in the low‐middle latitude, dayside zonal winds over a 5‐day period including the storm. Thus, we attribute the ionospheric changes to prompt penetration and meridional wind forcing, while ruling out the presence of a storm‐time disturbance dynamo and its related ionospheric effects.

Northern preference for terrestrial electromagnetic energy input from space weather

Terrestrial space weather involves the transfer of energy and momentum from the solar wind into geospace. Despite recently discovered seasonal asymmetries between auroral forms and the intensity of emissions between northern and southern hemispheres, seasonally averaged energy input into the ionosphere is still generally considered to be symmetric. Here we show, using Swarm satellite data, a preference for electromagnetic energy input at 450 km altitude into the northern hemisphere, on both the dayside and the nightside, when averaged over season. We propose that this is explained by the offset of the magnetic dipole away from Earth’s center. This introduces a larger separation between the magnetic pole and rotation axis in the south, creating different relative solar illumination of northern and southern auroral zones, resulting in changes to the strength of reflection of incident Alfvén waves from the ionosphere. Our study reveals an important asymmetry in seasonally averaged electromagnetic energy input to the atmosphere. Based on observed lower Poynting flux on the nightside this asymmetry may also exist for auroral emissions. Similar offsets may drive asymmetric energy input, and potentially aurora, on other planets.

Effect of auroral substorms on the ionospheric range spread-F enhancements at high southern midlatitudes using real time vertical-sounding ionograms

A comprehensive study has been undertaken on the effect of magnetic substorm onsets (as deduced from the auroral hourly electrojet AE-index ) on the occurrence of high midlatitude (or sub-auroral latitude) ionospheric range spread-F (Sr). Unlike the previous reports real-time ionograms were used in this analysis thus eliminating ambiguities stemming from the correlating secondary evidence of spread-F with auroral substorms. The Australian southernmost ionosonde station Hobart (51.6°S geom.) proved to be uniquely suitable for the task as being sufficiently close to the southern auroral zone. Sr was assigned in km to each hourly nighttime ionogram at two sounding frequencies: Sr1 (at 2MHz) and Sr2 (at 6MHz) for four months in 2002: January and June (representing southern summer and winter solstices), and March and September (representing autumn and vernal equinoxes). It is evident that the southern winter solstitial period (June) is associated with high endemic midlatitude spread-F activity. All other seasons are closely linked with temporal sequences of enhanced spread-F activity following substorm onsets.For the first time it was possible not only find a simultaneous occurrence pattern of these diverse phenomena but to deduce numerical characteristics of the response of midlatitude ionosphere to the global auroral stimulus. Excellent case events, hitherto unpublished, are shown illustrating the presence of the AE peaks (in nT) being ahead of Sr peaks (in km) by a time shift ∆t (in h). Sr1 magnitude showed a significant correlation with the magnitudes of the preceding AE with a correlation coefficient (r) of 0.51 (probability of the occurrence by chance less than 0.01). Sr2 peaks were more sensitive to auroral disturbances but were not correlated with the AE magnitude variations. The time shift (∆t) was on average 4h with a standard deviation of 3h.The general pattern in the occurrence of magnetic substorms and spread-F is very similar. A number of corresponding peaks of the AE-index and Sr fluctuations were identified. The sub-auroral ionosphere response tends to last longer than the initiating auroral disturbance (hysteresis effect). The simultaneous quiescent periods in the auroral and sub-auroral ionospheres have been encountered on at least 14 days.

Interhemispheric comparison of GPS phase scintillation at high latitudes during the magnetic-cloud-induced geomagnetic storm of 5–7 April 2010

Abstract. Arrays of GPS Ionospheric Scintillation and TEC Monitors (GISTMs) are used in a comparative scintillation study focusing on quasi-conjugate pairs of GPS receivers in the Arctic and Antarctic. Intense GPS phase scintillation and rapid variations in ionospheric total electron content (TEC) that can result in cycle slips were observed at high latitudes with dual-frequency GPS receivers during the first significant geomagnetic storm of solar cycle 24 on 5–7 April 2010. The impact of a bipolar magnetic cloud of north-south (NS) type embedded in high speed solar wind from a coronal hole caused a geomagnetic storm with maximum 3-hourly Kp = 8- and hourly ring current Dst = −73 nT. The interhemispheric comparison of phase scintillation reveals similarities but also asymmetries of the ionospheric response in the northern and southern auroral zones, cusps and polar caps. In the nightside auroral oval and in the cusp/cleft sectors the phase scintillation was observed in both hemispheres at about the same times and was correlated with geomagnetic activity. The scintillation level was very similar in approximately conjugate locations in Qiqiktarjuaq (75.4° N; 23.4° E CGM lat. and lon.) and South Pole (74.1° S; 18.9° E), in Longyearbyen (75.3° N; 111.2° E) and Zhongshan (74.7° S; 96.7° E), while it was significantly higher in Cambridge Bay (77.0° N; 310.1° E) than at Mario Zucchelli (80.0° S; 307.7° E). In the polar cap, when the interplanetary magnetic field (IMF) was strongly northward, the ionization due to energetic particle precipitation was a likely cause of scintillation that was stronger at Concordia (88.8° S; 54.4° E) in the dark ionosphere than in the sunlit ionosphere over Eureka (88.1° N; 333.4° E), due to a difference in ionospheric conductivity. When the IMF tilted southward, weak or no significant scintillation was detected in the northern polar cap, while in the southern polar cap rapidly varying TEC and strong phase scintillation persisted for many hours. This interhemispheric asymmetry is explained by the difference in the location of solar terminator relative to the cusps in the Northern and Southern Hemisphere. Solar terminator was in the immediate proximity of the cusp in the Southern Hemisphere where sunlit ionospheric plasma was readily convected into the central polar cap and a long series of patches was observed. In contrast, solar terminator was far poleward of the northern cusp thus reducing the entry of sunlit plasma and formation of dense patches. This is consistent with the observed and modeled seasonal variation in occurrence of polar cap patches. The GPS scintillation and TEC data analysis is supported by data from ground-based networks of magnetometers, riometers, ionosondes, HF radars and all-sky imagers, as well as particle flux measurements by DMSP satellites.

Auroral electrojet AL index and polar magnetic disturbances in two hemispheres

Earlier we found that the auroral electrojet AL index, indicating substorm activity in the northern hemisphere, in local summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap; we explained this effect as a result of interhemispheric field‐aligned currents, which suppress substorm field‐aligned currents in the summer hemisphere but increase these currents in the winter hemisphere. In the present paper, we took into account this effect and examined a method for reliably monitoring the substorm auroral electrojet, measured with the auroral electrojet AL index, by using hourly averages of geomagnetic field from two polar observatories (Thule and Vostok) in two hemispheres. We tested this method for 3 years. The correlation between the predicted and actual AL indices for these years was stable and very high, and it showed no significant dependence on season and a relatively weak UT variation. The correlation coefficient between the predicted and actual AL indices for these three years was about 0.89. The proposed method, based on using magnetic field data from two polar geomagnetic observatories in two hemispheres, not only significantly improves the reliability of monitoring the westward auroral electrojet in the northern hemisphere but it may also be used for monitoring the westward auroral electrojet in the southern hemisphere where no AL index is available because a significant portion of the southern auroral zone is located over the oceans. The results of this paper show that measurements of geomagnetic field in two hemispheres are of high importance for reliably monitoring the geomagnetic activity and related events in each hemisphere.

Some Features of Geomagnetic Micropulsations Observed During the Recent Quiet Solar Years, with Particular Reference to Data Obtained at the Near Conjugate Stations of Great Whale River and Byrd

Summary In dedicating this paper to Professor Sydney Chapman, the authors have taken the opportunity of describing some of the salient features of the early history of geomagnetic micropulsations since 1918, which were responsible for the Canadian interest in this field and the type of equipment still in use at many ground stations. In a sense, this is a review paper of the more recent work in this field, thus excluding that covered by other reviews such as that of Troitskaya (1964) and Kato, Saito and other workers in Japan. The importance of observations at polar stations is touched upon in relation to similar observations at mid-latitude stations during the recent years of quiet solar conditions when the 27-day solar rotation period is often prominent. This period is especially well-marked at the two near-conjugate stations, Byrd and Great Whale River, in the southern and northern auroral zones. The correlation between these two stations is only briefly mentioned, since it will be the subject of a separate report. The paper concludes with a brief discussion of the very important results of satellite observations. Better co-ordination of satellite data with that received on the ground is needed and internationally planned operations of land stations for the coming years of greater solar activity are necessary.

Magnetoconjugate phenomena in Alaska and Macquarie Is., Australia in 2003: position of the global maximum iso-aurorae

Abstract. An extensive magnetometer network in Alaska and Canada (consisting of 6 auroral and sub-auroral stations) and at Macquarie Is. (Australia) was used to study magnetoconjugate phenomena in 2003, using the H-component magnetograms. Altogether 193 magnetically disturbed days (with ΣKp≥10) were used in the analysis. The maximum negative swing in the H-component (ΔH) was assigned for each day from the auroral conjugate station's data. Two types of magnetoconjugate data were found: Type-1 when the daily (00:00–24:00 h UT) substorm activity was faithfully reproduced in the H-component variations in the northern and southern auroral zones, and Type-2 when a major peak in disturbance was largely missing in one conjugate location. A distinct maximum in the occurrence of the Type-1 events was in the southern summer (northern winter), reaching almost 70 per cent of cases. A minimum in this type of events was in the southern autumn and winter (around 40%). The correlation between ΣKp and ΔH for auroral stations faithfully reproduced the percentage occurrence of the Type-1 events for various seasons, with the maximum correlation coefficient r≈0.8 in summer. The time conjugacy of the events (i.e. the events occurring with a small time displacement in the southern and northern auroral zones) was highest for the auroral stations located close to the theoretical conjugate point location at L-value ≈ 5 (i.e. College and Macquarie Is.). The substorm onsets started earlier at the stations positioned east of the conjugate point of Macquarie Is. in spring and winter, indicating a westward drift of auroral disturbance with velocities of 1.4–1.6 km/s. The magnitude of average ΔH increased rapidly past the invariant latitude of Macquarie Is. towards the latitude of its nearest northern conjugate station College , particularly in spring. The average level of disturbance was consistently lower by about 30% at Macquarie Is. than at College. Since Macquarie Is. is the only available auroral station in the southern auroral zone the maximum southern auroral activity level (maximum iso-aurorae) should be inferred from magnetic data obtained at a northern conjugate station positioned at a slightly higher L-value, such as College (L-value = 5.73). It is concluded from the above considerations that the maximum iso-aurorae is located at a shell with L-value ≥ 5.73 in both hemispheres.

A statistical study of large‐amplitude parallel electric fields in the upward current region of the auroral acceleration region

We present a survey of 64 direct observations of large‐amplitude parallel electric fields E∥ in the upward current region of the southern auroral acceleration zone, obtained by the three‐axis electric field experiment on Polar. These E∥ events range in amplitude from about 25 to 300 mV/m and represent a significant fraction of the total electric field strength (E∥/E⟂ ranges from ∼0.25 to O(10)). The E∥ structures, which tend to occur at the edges of oppositely directed (converging) pairs of perpendicular electric field structures (electrostatic shocks), have transverse (to the magnetic field) widths of ∼1.0–20 km at altitudes ranging from 0.8RE to 1.5RE, assuming the structures are stationary. The parallel potential drops associated with these large‐amplitude parallel electric fields are highly localized in altitude (e.g., tens of kilometers as opposed to thousands of kilometers). The amplitude of the parallel electric field shows a strong anticorrelation with the plasma density inferred from spacecraft potential measurements. We find no apparent correlation between the amplitude of the parallel electric field and altitudes sampled (between 0.8RE and 1.5RE), current density, and Kp, though there is a suggestion that the largest E∥/E⟂ ratios are confined to lower altitudes. Taking sampling biases into consideration, we find that the large parallel electric fields occur preferentially at higher values of Kp and within a thin layer centered about 1.28RE. A detailed analysis favors ambipolar effects over electron inertial effects as an explanation for the parallel electric field signatures.

Magnetic component of narrowband ion cyclotron waves in the auroral zone

We report observations of waves near the local proton cyclotron frequency and its lowest harmonics, made by the Plasma Wave Instrument on board the Polar spacecraft, on orbits for which the perigee (at an altitude of 1 Earth radius) was in or near the southern auroral zone. The electromagnetic nature of these waves was revealed by measuring their magnetic components simultaneously with two independent antenna systems, one a single loop and the other a set of triaxial search coils. Waves of this kind were found in the southern auroral zone on about a third of the orbits examined. Peaks in the magnetic field power spectra occurred at frequencies both below and above the fundamental cyclotron frequency. The ratio of the amplitudes of the electric and magnetic fluctuations was usually greater than the speed of light, suggesting that we observe a magnetic component related to the wave mode customarily called electrostatic ion cyclotron waves. The fluctuating vector of the wave magnetic field was in or close to the plane perpendicular to the static magnetic field. Our main result is that its polarization, which covered a broad range from left‐hand elliptic to right‐hand elliptic, can be explained by the superposition of many linearly polarized waves.

A statistical comparison of the outflow of [formula omitted], NO + and [formula omitted] molecular ions with that of atomic O + ions using Polar/TIMAS observations

Ion composition data from the first 22 months of operation (until the end of 1997) of the Polar/TIMAS instrument, covering the 15-eV/e to 33-keV/e energy range, have been surveyed to determine the typical abundance of N + 2 , NO + and O + 2 ions as a group ( M/Q∼30) in the auroral ion outflow, as compared to that of the better known O + ions. The results, obtained at and near solar minimum, indicate that molecular ions have roughly the same energy distribution as the O + ions, with maximum differential flux occurring below 400 eV, but are far less abundant, by two orders of magnitude on average. The molecular ions also differ from the O + ions in that they seem more specifically associated with enhanced geomagnetic activity, possibly with some electrodynamic threshold being surpassed in the ionosphere. The most intense fluxes of molecular ions, 10 5–10 6 (cm 2 s ster keV/e) −1, are usually encountered near the Polar perigee in the southern auroral zone (at R ∼ 2 R E), but the longer lasting encounters, 30 min or more, are closer to the Polar apogee in the northern hemisphere, often well inside the polar cap (at R ∼ 4–9 R E). The molecular ions are observed at varying local time but too infrequently to yield a significant measure of local time distribution.

Direct observation of large, quasi‐static, parallel electric fields in the auroral acceleration region

The three‐axis electric field experiment on the Polar satellite provides direct observations of electric field components parallel and perpendicular to the local magnetic field with no arbitrary adjustment parameters. Approximately 750 perigee passes through each of the two southern auroral zones at a geocentric altitude of about two Earth radii have been computer‐searched for parallel electric fields whose eight point (0.2 or 0.4 second) average exceeded 100 mV/m. After elimination of spurious events due to shadowing, saturation, and ten other effects, four events containing parallel fields of 200–300 mV/m, remain. These four events all occur in upward field aligned current regions, their parallel electric fields are all positive such that . >0, and they occur at boundaries between regions of active and quiet perpendicular electric fields. Up‐going ion beams are observed in the active field regions, and the plasma density is higher in the quiet field regions than in the adjacent active field regions. These boundaries are interpreted in terms of model equipotentials, some of which are below the spacecraft in the large field regions and all of which are above the spacecraft in the quiet field regions. In this model, the expected location of large parallel electric fields is where they are observed. That the potential difference measured by the electric field instrument along the vehicle trajectory and the kinetic energy of the up‐going ions are equal lends further credence to the data and its interpretation.

Early rocket observations of auroral Bremsstrahlung and its absorption in the mesosphere

In the summer and autumn of 1957 the author and his colleagues at the University of Iowa conducted 10 successful balloon‐launched rocket (rockoon) flights of Geiger‐Mueller tubes during two shipboard expeditions, one to the arctic and the other to the antarctic. Summit altitudes ranged from 77 to 130 km. One flight was conducted in the equatorial zone, six in the northern auroral zone, and three in the southern auroral zone. Detailed results of each flight are presented. Auroral bremsstrahlung was detected on eight of the nine high‐latitude flights. By virtue of physical shielding, the detectors were insensitive to photons of energy E < 8 keV. Hence our derived absolute spectra refer to the high‐energy tail of the auroral electron spectrum and the resulting bremsstrahlung. For the energy range E > 8 keV, we find a typical e‐folding energy E* of about 10 keV for a differential photon number spectrum of the form dn/dE = A exp(−E/E*) with A in units of photons (cm² s keV)−1. Essentially, the same value of E* is obtained by two complementary methods: (1) The dependence of counting rate on atmospheric depth Q and (2) the inverse ratio of counting rates of an “unshielded” tube and one with an added shield. A detailed discussion of the bases for interpretation of the flight data is given in an extended appendix. For a representative set of data (flight 64) we find an omnidirectional flux of downward moving photons of 2.4×104 (cm² s)−1 at Q = 0, the integral of the above spectrum from E = 0 to E = ∞ with the explicit understanding that this result does not include the presumably much greater flux of photons having E < 8 keV and a much steeper spectrum. The corresponding electron number flux striking the top of the atmosphere is 9.3×107 electrons (cm² s)−1 with an e‐folding energy T* = 30 keV. The corresponding energy flux is 4.5 erg (cm² s)−1. Both of the two latter fluxes represent the integral from electron energy T = 0 to T = ∞ of the high‐energy tail of the electron spectrum, again with the explicit understanding that the estimates do not include the steeply rising spectrum for T ≲ 15 keV.

The effect of the impact of comet Shoemaker Levy‐9 on Jupiter's aurorae

We present infrared spectra and images of the jovian aurorae taken at wavelengths sensitive to the H3+ molecular ion during the period around the impact of Comet Shoemaker Levy‐9. The spectra were obtained using CGS4 on the United Kingdom Infrared Telescope and the images using NSFcam on NASA's Infrared Telescope Facility. Comparison with spectra obtained in May, 1993, shows that while the relative intensities of the northern and southern auroral zones prior to and during impact week (July 16–22, 1994) were broadly comparable with those of 1993, a few days after the last collision the northern aurora was considerably enhanced and its southern counterpart somewhat depressed. The north/south auroral ratio was returning to more normal values a week later. The effect of material drifting from the impact sites to the southern auroral zone is discussed in relation to these results.

Mean thermospheric winds observed from Halley, Antarctica

Abstract. Thermospheric winds on a total of 237 nights have been studied for the effects due to geomagnetic activity, solar flux, and season. The observations have been made from 1988 to 1992 by a Fabry-Perot interferometer (FPI) operating at Halley (75.5°S, 26.6°W), Antarctica. This is the first statistical study of thermospheric winds near the southern auroral zone. The main factor affecting the wind velocities is the geomagnetic activity. Increases in activity cause an increase in the maximum equatorward wind, and cause the zonal wind in the evening to become more westward. Smaller changes in the mean wind occur with variations in season and solar flux. The small variation with solar flux is more akin to the situation found at mid-latitudes than at high latitudes. Since the geomagnetic latitude of Halley is only 61°, it suggests that the variability of the wind with solar flux may depend more on geomagnetic than geographic latitude. These observations are in good agreement with the empirical Horizontal Wind Model (HWM90). However, comparisons with predictions of the Vector Spherical Harmonic Model (VSH) show that for low geomagnetic activity the predicted phases of the two components of the wind closely resemble the observations but the modelled amplitudes are too small by a factor of two. At high geomagnetic activity the major differences are that modelled zonal velocity is too westward in the evening and too eastward after 04 UT. The modelled ion densities at the F-region peak are a factor of up to 9 too large, whilst the predicted mean value and diurnal variation of the altitude of the peak are significantly lower than those observed. It is suggested that these differences result from the ion loss rate being too low, and an inaccurate model of the magnetic field.

Auroral x-ray imaging from high- and low-Earth orbit

Observations of bremsstrahlung x rays emitted by energetic electrons impacting the Earth's atmosphere can be used for remotely sensing the morphology, intensity, and energy spectra of electron precipitation from the magnetosphere. The utility of the technique is derived from the broad energy range of observable x rays (2 to > 100 KeV), the simple emission process, the large x-ray mean free path in the atmosphere, and negligible background. Two auroral x-ray imagers, developed for future spaceflights, are discussed. The Polar Ionospheric X-Ray Imaging Experiment is scheduled for launch on the NASA International Solar-Terrestrial Physics/Global Geospace Science program POLAR satellite in 1994. The POLAR orbit, with an apogee and perigee of 9 and 1.8 R[sub e] (Earth radii), respectively, affords the opportunity to image the aurora from a high altitude above the north pole continuously for several hours. The Magnetospheric Atmospheric X-Ray Imaging Experiment (MAXIE) was launched aboard the NOAA-I satellite on August 8, 1993. The 800-km polar orbit passes over both the northern and southern auroral zones every 101 min. MAXIE will be capable of obtaining multiple images of the same auroral region during a single satellite orbit. The experimental approaches used to exploit these very different orbits for remotemore » sensing of the Earth's auroral zones are emphasized.« less

Evidence of auroral plasma cavities at Uranus and Neptune from radio burst observations

Radio bursts originating from the stronger magnetic polar regions of both Uranus and Neptune were detected by the planetary radio astronomy experiment during the Voyager 2 encounters with the planets. It has previously been demonstrated that these bursts are beamed into a broad, hollow emission pattern from their auroral sources. We now show that the bursts at both planets also manifest similar detailed patterns, with the waves beamed into two separate and distinct radiation cones at intermediate wave frequencies. This double‐cone emission pattern is predicted by relativistic cyclotron resonance theory, and application of this theory to the observed emission pattern yields the plasma density structure within the radio source region. Calculations indicate that at both Uranus and Neptune the plasma‐to‐cyclotron frequency ratio can drop well below 0.01 within the active region. Such low values indicate that the southern auroral zones at both planets contain an auroral plasma cavity that is similar to that found in Earth's nightside auroral zone.

H3(+) fundamental band in Jupiter's auroral zones at high resolution from 2400 to 2900 inverse centimeters

Following the previous detection of H3(+) in the southern auroral zone of Jupiter from its 2nu2 band, a search was made for the fundamental at 4 microns. Up to 42 lines of this band were detected in emission, at high resolution, on the auroral spot of each hemisphere. A rotational temperature was derived for the southern and northern zones, respectively, of 1000 + or - 40K and 835 + or - 50 K. The intensity of the lines was on the average two times stronger in the south than in the north. The 2nu2 band, which was sought in the north only on this occasion, was not detectable. A purely thermal mechanism for the H3(+) production is implied. Spatial extension and temporal variability of the excitation is discussed. 20 refs.

Cyclotron maser radiation from planetary magnetospheres (abstract)

For over 20 years it has been known that the Earth is an intense radio emitter in the frequency range from about 100 to 500 kHz. This radiation is generated at altitudes of about one Earth radius over the northern and southern auroral zones, and is closely correlated with the occurrence of discrete auroral arcs. During active periods, the total radiated power is very large, averaging from 107 to 108 W, with peaks possibly as high as 109 W. It is now widely believed that this radiation is generated by a process known as a cyclotron maser instability. This process involves a Doppler-shifted cyclotron resonance interaction between the free-space R-X mode and the auroral electrons. The most recent data, from the Viking satellite, indicate that the free-energy source driving the instability is produced by electrons trapped by the magnetic mirror force and the electric field response for the auroral particle acceleration. In addition to the Earth, it is now known that four other planetary magnetospheres, at Jupiter, Saturn, Uranus, and Neptune, all have similar types of radio emissions. Also, it has been suggested that certain types of microwave emissions from the Sun, and from binary star systems may also be produced by this same mechanism. Thus, in addition to being a relatively new, interesting plasma physics phenomenon, the cyclotron maser mechanism appears to be important for a broad range of astronomical radio sources.

Temperatures of the Jovian auroral zone inferred from 2-μm H 2 quadropole line observations

We have observed 2-μm H 2 quadropole lines, S 1(0), S 1(1), and S 1(2), in the southern auroral zone of Jupiter. From the observed intensities, we derived a rotational temperature of 730°K plus 490°K and minus 200°K in the 1 to 0.01 μbar pressure range. We conclude that the derived rotational temperature should represent the local neutral kinetic temperature. Furthermore, we found that H 2( ν = 1) population in the auroral zone is at least 500 times greater than the normal Boltzmann population, indicating that nonthermal emission is dominant for these lines. We also calculated both nonthermal and thermal intensities of the pure H 2 rotational quadropole lines and found the opposite result—that the thermal emission dominates over the nonthermal emission—for those lines in the longer wavelength region (>5 μm).

Interpretation of electron spectra in morningside pulsating aurorae

Energy spectra of precipitating electrons in morningside aurorae were obtained with instruments on the NOAA 6 satellite during several passes through the southern auroral zone in 1982. It is substantiated with the aid of coordinated data from particle detectors and a photometer in the Simulated Emission of Energetic Particles experiment payload on satellite S81‐1 that the precipitation observed by NOAA 6 was probably in a region of pulsating aurorae. The particle detectors and forward/downward looking quadrant photometer onboard S81‐1 all exhibited regular modulations at a period of about 5–15 s. The particle detectors on NOAA 6 showed fluctuating electron intensities over an energy range from several kiloelectronvolts to many tens of kiloelectronvolts, superimposed on a steady background component extending down to energies less than 1 keV. The NOAA 6 electron observations are in qualitative agreement with a time‐dependent precipitation model that includes the production of backscattered and secondary electrons from the atmosphere. In this model the time variations imposed on the precipitation are restricted to the energetic portion of the electron spectrum, while the integrating effects inherent in the production of backscattered and secondary electrons, together with phase mixing introduced as those electrons transit between conjugate hemispheres, insure the existence of a nearly time independent population of low‐energy electrons. A steady background precipitation is thereby created as a natural consequence of a time‐variable primary precipitation.