Auroral Heating Research Articles

Thermosphere–ionosphere coupling in response to recurrent geomagnetic activity

The paper presents the global thermosphere–ionosphere response to the high-speed solar wind streams and the subsequent recurrent geomagnetic variations with a period of 9d during the period of time 1 October 2007–31 March 2009. The COSMIC electron density at fixed heights, as well as the ionospheric parameters foF2 and hmF2, and the two coefficients characterizing the top and bottom side vertical gradients of the electron density profile, are used for investigating the ionospheric 9-d (s=0) wave response. The SABER temperature data are utilized for studying the response of the lower thermosphere to the recurrent auroral heating. The COSMIC and SABER measurements are analyzed by one and the same method where the atmospheric tides and planetary waves which are present in the temperature and electron density measurements are simultaneously extracted from the data. The use of such data analysis approach brings to light additional features of the ionospheric response to a recurrent geomagnetic activity which have not been found before.

Observations of AGW/TID propagation across the polar cap: a case study

Abstract. In this paper, we present observational evidence for the trans-polar propagation of large-scale Traveling Ionospheric Disturbances (TIDs) from their nightside source region to the dayside. On 13 February 2001, the 32 m dish of EISCAT Svalbard Radar (ESR) was directing toward the geomagnetic pole at low elevation (30°) during the interval 06:00–12:00 UT (MLT ≈ UT + 3 h), providing an excellent opportunity to monitor the ionosphere F-region over the polar cap. The TIDs were first detected by the ESR over the dayside north polar cap, propagating equatorward, and were subsequently seen by the mainland UHF radar at auroral latitudes around geomagnetic local noon. The propagation properties of the observed ionization waves suggest the presence of a moderately large-scale TIDs, propagating across the northern polar cap from the night-time auroral source during substorm conditions. Our results agree with the theoretical simulations by Balthazor and Moffett (1999) in which poleward-propagating large-scale traveling atmospheric disturbances were found to be self-consistently driven by enhancements in auroral heating.

Predicting global average thermospheric temperature changes resulting from auroral heating

[1] The total Poynting flux flowing into both polar hemispheres as a function of time, computed with an empirical model, is compared with measurements of neutral densities in the thermosphere at two altitudes obtained from accelerometers on the CHAMP and GRACE satellites. The Jacchia-Bowman 2008 empirical thermospheric density model (JB2008) is used to facilitate the comparison. This model calculates a background level for the “global nighttime minimum exospheric temperature,” Tc, from solar indices. Corrections to this background level due to auroral heating, ΔTc, are presently computed from the Dst index. A proxy measurement of this temperature difference, ΔTc, is obtained by matching the CHAMP and GRACE density measurements with the JB2008 model. Through the use of a differential equation, the ΔTc correction can be predicted from IMF values. The resulting calculations correlate very well with the orbit-averaged measurements of ΔTc, and correlate better than the values derived from Dst. Results indicate that the thermosphere cools faster following time periods with greater ionospheric heating. The enhanced cooling is likely due to nitric oxide (NO) that is produced at a higher rate in proportion to the ionospheric heating, and this effect is simulated in the differential equations. As the ΔTc temperature correction from this model can be used as a direct substitute for the Dst-derived correction that is now used in JB200, it could be possible to predict ΔTc with greater accuracy and lead time.

Effects of high-latitude thermosphere heating at various scale sizes simulated by a nonhydrostatic global thermosphere–ionosphere model

Using the University of Michigan Global Ionosphere–Thermosphere Model (GITM) variations of the high-latitude Joule and auroral heating with increasing spatial resolution and the resulting changes in the high-latitude thermosphere–ionosphere are studied. Additionally, the application of multiplicative factors artificially increasing the Joule heating in global thermosphere models is investigated by running simulations at low resolution, at high resolution, and then at low resolution again, with the Joule heating artificially increased to be similar to the value derived when running with high resolution. All simulations have been performed for December solstice conditions at constant low geomagnetic and moderate solar activity levels. Increasing the resolution from 5° longitude by 5° latitude to 2.5° longitude by 0.3125° latitude increases the high-latitude mean Joule heating by ∼ 40 % , partially by better capturing the peaks in the electric field. Although the Joule heating is increased, the global mean temperature is not altered significantly due to resulting adiabatic cooling. When the Joule heating is artificially increased in the low resolution simulations, the temperature increases more, due to the model inability to adequately capture the dynamical cooling. It is therefore suggested that artificially increasing the Joule heating may not be the best method for capturing the realistic temperature structure or the dynamics within the thermosphere.

Climatology of the stationary planetary waves seen in the SABER/TIMED temperatures (2002‐2007)

The paper focuses on the global spatial structure, seasonal and interannual variability of the stationary planetary waves (SPW) with zonal wavenumber s = 1 (SPW1) and s = 2 (SPW2) derived from the SABER/TIMED temperature measurements for 6 full years (January 2002–December 2007). The monthly mean climatological (multiyear average, 2002–2007) depictions of SPW temperature field extending between 20 and 120 km altitude and ±50° latitude are derived. A special attention is paid to the SPW temperature field in the lower thermosphere where the SPW1 amplifies in May and December–January in the northern hemisphere (NH) and in April–May and November in the southern hemisphere (SH). Amplitudes in both hemispheres are similar in strato‐mesosphere; however, in the lower thermosphere SH amplitudes are significantly larger (∼30 K) than those in NH (∼20 K). The NH SPW1 is able to propagate up to 115 km altitude contributing to the winter amplification of the lower thermospheric SPW1, but that from the SH cannot penetrate above ∼90–100 km. In the lower thermosphere (above h = 100 km) mainly trapped SPW signal is observed. The SPW1 are almost out of phase in both hemispheres; the average (2002–2007) phases are ∼100°W in NH and ∼120°E in SH. Strong evidence indicates that auroral heating is a main origin of the lower thermospheric SPW temperature field.

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

Synchronous <I>Nm</I>F2 and <I>Nm</I>E daytime variations as a key to the mechanism of quiet-time F2-layer disturbances

Abstract. The observed NmF2 and NmE variations were analyzed for the periods of positive and negative quiet-time F2-layer disturbances (Q-disturbances) observed in the midlatitude daytime F2-layer to specify the mechanism of their origin. The noontime δNmF2 and δNmE deviations demonstrate a synchronous type of variation which can be explained by vertical gas motion in the thermosphere. This neutral gas motion should result in atomic abundance variations, the latter being confirmed by the Millstone Hill ISR observations for periods of positive and negative Q-disturbance events. The analysis of the ISR data has shown that atomic oxygen concentration variations are the main cause of the daytime F2-layer Q-disturbances. The auroral heating which controls the poleward thermospheric wind is considered to be the basic mechanism for the Q-disturbances, however, the specific mechanisms of positive and negative Q-disturbances are different. Some morphological features of the Q-disturbances revealed earlier are explained in the scope of the proposed concept.

A global study of nighttime midlatitude topside spread echoes

Range spreading is frequently evident in midlatitude topside ionograms from the ISS‐B satellite. These ionograms provide a geographically unbiased perspective for the first global investigation of nighttime spread echo events that focuses solely on the midlatitude domain. The topside echo occurrence probability data show significant longitudinal variations, along with very large winter/equinox asymmetries in several geographic regions. The spread echoes are divided into two types and found to have different geophysical dependences. Small‐scale events that appear as spreading in one or two consecutive ionograms have a marked seasonal dependence on magnetic declination and no obvious correlation with auroral indices. Larger‐scale events in which the spreading is observed over spatial domains of 1000 km or more have less dependence on magnetic declination but are anticorrelated with the AE index in periods 6–8 hours prior to their appearance. The declination dependence for the small‐scale events suggests that the irregularities are most prevalent when the E region conductivity is low at both ends of the flux tube. This in turn suggests that electric fields mapping from the E region up to F region altitudes are likely involved in their generation. The correlation of the larger‐scale events with depressed AE signatures suggests that traveling ionospheric disturbances caused by auroral heating are not the most likely cause of these large features at midlatitudes.

A two-dimensional simulation of thermospheric vertical winds in the vicinity of an auroral arc

Abstract The observations made by Fabry-Perot interferometers (FPIs), radars, and satellites have indicated that large vertical motion in the polar region is occasionally generated in the thermosphere associated with auroral activities. However, the behavior of the vertical wind is often very complicated, and the cause of the vertical wind has not been explained by auroral heating or by ion-neutral drag alone. It has been pointed out that a background horizontal flow is likely to significantly alter the dynamics of the neutral atmosphere near an auroral arc. Recent observations have also suggested that strong downward motion is generated in the vicinity of an auroral arc. To study the thermospheric dynamics near a local heating region embedded in a large-scale horizontal flow, a two-dimensional numerical simulation of the thermospheric dynamics has been performed. It is found that interaction of local heating and strong horizontal flow could play an important role in generating vertical motion near an auroral arc. The simulation results indicate that for a horizontal wind speed larger than about 300 m/s, a steady wave-like structure of the neutral wind is formed within and downstream of the heated region. For a horizontal wind speed less than about 300 m/s, on the other hand, no significant vertical motion is generated outside the heated region. This process might account for at least some of the observed features of vertical motion within and outside an auroral arc.

On the response of the exospheric temperature to the auroral heating impulse during geomagnetic disturbances

On the basis of an assumed impulse-type energy input at auroral oval due to the particle precipitation, the response of the exospheric temperature T ∞ has been investigated in terms of changes in input energy, geomagnetic indices and latitude under simplified assumptions It is shown that the resulting T ∞ is proportional to the peak heating rate and inversely proportional to the gradient of the atmospheric density. Even during low solar activity and quiet geomagnetic conditions, the precipitated electron energy density in the thermosphere is found in the range of about 1–1.5×10 −6 erg/cm 3 s, reaching to about 3×10 −6 erg/cm 3 s at 2000 K and is easily supported by the solar wind–magnetospheric interaction. The auroral energy inputs lasting less than 2 h or so hardly generate geomagnetic storms. The inhomogeneous energy equation has been solved in horizontal two dimensions in presence of horizontal heat conduction, to study the T ∞ at other latitudes and its time delay with respect to the peak impulse heating time. This time delay is independent of the storm intensity, i.e., proxy geomagnetic indices and is about 6–7 h at low and mid latitudes and about 3 h or less at high latitudes in agreement with recent coupled magnetospheric–ionospheric–thermospheric models and with the available observed values. The diffusion coefficient of horizontal heat conduction deduced is about 7×10 4 m 2/s. The role of the horizontal heat conduction in the energy budget needs further investigation.

The global ionosphere–thermosphere model

The recently created global ionosphere–thermosphere model (GITM) is presented. GITM uses a three-dimensional spherical grid that can be stretched in both latitude and altitude, while having a fixed resolution in longitude. GITM is nontraditional in that it does not use a pressure-based coordinate system. Instead it uses an altitude-based grid and does not assume a hydrostatic solution. This allows the model to more realistically capture physics in the high-latitude region, where auroral heating is prevalent. The code can be run in a one-dimensional (1-D) or three-dimensional (3-D) mode. In 3-D mode, the modeling region is broken into blocks of equal size for parallelization. In 1-D mode, a single latitude and longitude is modeled by neglecting any horizontal transport or gradients, except in the ionospheric potential. GITM includes a modern advection solver and realistic source terms for the continuity, momentum, and energy equations. Each neutral species has a separate vertical velocity, with coupling of the velocities through a frictional term. The ion momentum equation is solved for assuming steady-state, taking into account the pressure, gravity, neutral winds, and external electric fields. GITM is an extremely flexible code—allowing different models of high-latitude electric fields, auroral particle precipitation, solar EUV inputs, and particle energy deposition to be used. The magnetic field can be represented by an ideal dipole magnetic field or a realistic APEX magnetic field. Many of the source terms can be controlled (switched on and off, or values set) by an easily readable input file. The initial state can be set in three different ways: (1) using an ideal atmosphere, where the user inputs the densities and temperature at the bottom of the atmosphere; (2) using MSIS and IRI; and (3) restarting from a previous run. A 3-D equinox run and a 3-D northern summer solstice run are presented. These simulations are compared with MSIS and IRI to show that the large-scale features are reproduced within the code. We conduct a second equinox simulation with different initial conditions to show that the runs converge after approximately 1.5 days. Additionally, a 1-D simulation is presented to show that GITM works in 1-D and that the dynamics are what is expected for such a model.

Estimated energy balance in the jovian upper atmosphere during an auroral heating event

We present an analysis of a series of observations of the auroral/polar regions of Jupiter, carried out between September 8 and 11, 1998, making use of the high-resolution spectrometer, CSHELL, on the NASA InfraRed Telescope Facility (IRTF), Mauna Kea, Hawaii; these observations spanned an “auroral heating event.” This analysis combines the measured line intensities and ion velocities with a one-dimensional model vertical profile of the jovian thermosphere/ionosphere. We compute the model line intensities both assuming local thermodynamic equilibrium (LTE) and, relaxing this condition (non-LTE), through detailed balance calculations, in order to compare with the observations. Taking the model parameters derived, we calculate the changes in heating rate required to account for the modelled temperature profiles that are consistent with the measured line intensities. We compute the electron precipitation rates required to give the modelled ion densities that are consistent with the measured line intensities, and derive the corresponding Pedersen conductivities. We compute the changes in heating due to Joule heating and ion drag derived from the measured ion velocities, and modelled conductivities, making use of ion-neutral coupling coefficients derived from a 3-D global circulation model. Finally, we compute the cooling due to the downward conduction of heat and the radiation-to-space from the H 3 + molecular ion and hydrocarbons. Comparison of the various heating and cooling terms enables us to investigate the balance of energy inputs into the auroral/polar atmosphere. Increases in Joule heating and ion drag are sufficient to explain the observed heating of the atmosphere; increased particle precipitation makes only a minor heating contribution. But local cooling effects—predominantly radiation-to-space—are shown to be too inefficient to allow the atmosphere to relax back to pre-event thermal conditions. Thus we conclude that this event provides observational, i.e. empirical, evidence that heat must be transported away from the auroral/polar regions by thermally or mechanically driven winds.

Jupiter Thermospheric General Circulation Model (JTGCM): Global structure and dynamics driven by auroral and Joule heating

A growing multispectral database plus recent Galileo descent measurements are being used to construct a self‐consistent picture of the Jupiter thermosphere/ionosphere system. The proper characterization of Jupiter's upper atmosphere, embedded ionosphere, and auroral features requires the examination of underlying processes, including the feedbacks of energetics, neutral‐ion dynamics, composition, and magnetospheric coupling. A fully 3‐D Jupiter Thermospheric General Circulation Model (JTGCM) has been developed and exercised to address global temperatures, three‐component neutral winds, and neutral‐ion species distributions. The domain of this JTGCM extends from 20‐μbar (capturing hydrocarbon cooling) to 1.0 × 10−4 nbar (including auroral/Joule heating processes). The resulting JTGCM has been fully spun‐up and integrated for ≥40 Jupiter rotations. Results from three JTGCM cases incorporating moderate auroral heating, ion drag, and moderate to strong Joule heating processes are presented. The neutral horizontal winds at ionospheric heights vary from 0.5 km/s to 1.2 km/s, atomic hydrogen is transported equatorward, and auroral exospheric temperatures range from ∼1200–1300 K to above 3000 K, depending on the magnitude of Joule heating. The equatorial temperature profiles from the JTGCM are compared with the measured temperature structure from the Galileo ASI data set. The best fit to the Galileo data implies that the major energy source for maintaining the equatorial temperatures is due to dynamical heating induced by the low‐latitude convergence of the high‐latitude‐driven thermospheric circulation. Overall, the Jupiter thermosphere/ionosphere system is highly variable and is shown to be strongly dependent on magnetospheric coupling which regulates Joule heating.

A proposal to improve the CIRA '86 model in the equatorial region: the ddMSIS model

The paper intends to draw the attention of future authors of the planned CIRA model to two effects discovered by us previously. They should be taken into account both in the description of the total density and in the physical connection between the magnetosphere and the neutral upper atmosphere. The first is the nighttime density maximum – even in geomagnetically quiet periods – which is probably connected with compressional heating due to global winds originating from the subsolar point. The second is the dependence of the total density residuals (with respect to the CIRA '86 model values) on D st, which might be the consequence of heating by particles precipitating from the ring current [Nabl. ISZ, vol. 24 (1986) pp. 121–127]. This effect hints at an additional magnetospheric heating (besides the well-known auroral heating in the main phase of geomagnetic storms) which acts during the recovery phase of the storms as well with a maximum near the equator. The equatorial source may be connected with ions, neutralized by charge exchange (Energetic Neutral Atoms, ENA), precipitating from the ring current and having a velocity vector directed towards the Earth. A new multiplicating function to the CIRA '86 model (ddMSIS model) describes both additional heating effects.

Dynamical coupling of neutrals and ions in the high‐latitude F region: Simultaneous FPI and HF radar observations at Syowa Station, Antarctica

Simultaneous measurements of neutral winds and plasma drifts have been carried out to clarify the local neutral‐ion coupling processes in the auroral F region. High‐resolution maps of thermospheric winds and plasma drifts were made using data obtained by the Fabry‐Perot Doppler Imaging System (FPDIS) and by the Syowa‐South HF radar, respectively, both installed at Syowa Station (66.4° in magnetic latitude), Antarctica during the solar minimum period in 1996. We made case studies for two nights: one on 13 May 1996 during a rather quiet period including the growth phase of a substorm, and the other on 24–25 April 1996 during a moderately active period. From 2000 to 2200 UT on 13 May, the velocities of neutral winds and plasma drifts showed similar time variations: a negative excursion followed by a positive excursion. However, the variations of plasma drifts preceded those of neutral winds with a time lag of ∼30 min, and plasma drift velocities were about 7–10 times greater than neutral wind velocities. Therefore, it is suggested that the variations of neutral winds were driven by the ion drag force. To interpret results of the 13 May case, we simulated the relationship between neutral winds and ion drifts by including the effect of auroral heating, such as Joule and particle heating. The results suggest that strong neutral winds were driven by the ion drag and the pressure gradient force induced by auroral heating. In contrast, in the case of 24–25 April, no similarity was found between neutral winds and plasma drifts. Although it is difficult to specify sources for the discrepancy, some possible origins are proposed. For example, if the pressure gradient force is much greater than the ion drag force as in the case of disturbed period, neutral wind variations become independent of plasma drift variations. The discrepancy is also caused by the existence of vertical winds associated with active aurora.

Secular variability of the geocoronal Balmer‐alpha brightness: magnetic activity and possible human influences

Measurements of the geocoronal Balmer‐alpha (Hα) brightness taken between 1983 and 1994 at the Arecibo Observatory (18.35° N, 66.75° W) occasionally display aperiodic brightness variations that we attribute to a local response of exospheric hydrogen abundance to geomagnetic activity. Approximately twofold Hα brightness enhancements in the days following moderate storm onset conditions are demonstrated, and the absolute brightness during these events is significantly greater than modeled brightness based on quiet geomagnetic conditions. Although there are no direct measurements of the line center solar Lyman beta (Ly β) flux that pumps Hα during these events, the magnitude of the brightness enhancements and the temporal reproducibility of the phenomena (following onset of all storms in our data) make it likely that the enhanced Hα brightness is due to enhanced column abundances of hydrogen in the exosphere above Arecibo and not to sporadic increases in the solar Ly β flux. Increased hydrogen abundance due to thermal expansion of the midlatitude atmosphere following propagation of the auroral heating event cannot account for the entire enhancement. The data also suggest that Hα. brightness has become systematically stronger from 1984 to 1994, independent of solar cycle variability or variations due to geomagnetic activity. An increase of ∼ 3% per year, may be attributable to real change in the geocoronal hydrogen column abundance during the period, and is possibly a consequence of increasing hydrogenous species concentrations in the lower atmosphere. The enhancement of geocoronal Hα brightness between 1984 and 1994 may be the consequence of increasing methane in the troposphere and may signal that the effects of methane deposition in the lower atmosphere have propagated throughout the atmosphere.

A self‐consistent model of the Jovian auroral thermal structure

A one‐dimensional (1‐D) model coupling a two‐stream electron transport model of energy deposition with a 1‐D thermal conduction model has been developed. It is applied to investigate the links between auroral heat input and the vertical temperature of Jupiter's upper atmosphere. Two energy distributions meant to reproduce the emissions of a diffuse and a discrete aurora are used to evaluate the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. The values of observable quantities such as the altitude of the H2 emission peak, thermal infrared (IR), ultraviolet (UV) emissions, and temperatures associated with various optical signatures are used to constrain the parameters of these distributions. It is shown that the high‐energy component of these energy distributions heats a region of the homosphere between 10−4 and 10−6 bar and mainly controls the H2 temperature and the far‐UV (FUV) emission. A 3‐keV soft electron component is necessary to heat the region directly above the homopause, between 10−6 and 10−9 bar. It has a large influence on the H2 and H3+ temperatures and on the H3+ near‐IR (NIR) emission. It is used in conjunction with a weak 100 eV component which is responsible for heating the thermosphere, from 10−9 to 10−12 bar and exerts a control on the exospheric temperature. The calculated temperatures, UV, and IR emissions suggest that the model probably misses a nonparticle heat source in the 10−5 bar region, that is expected to balance the strong hydrocarbon cooling. Sensitivity tests are performed to evaluate the importance of the parameters of the energy distributions. They show that the FUV color ratio increases with the characteristic energy (or high‐energy cutoff) of the high‐energy component, while the H2 rovibrational temperature varies inversely. A trade‐off is therefore necessary for these two parameters to simultaneously meet their observational constraints. Further tests demonstrate the essential thermostatic role played by H3+, which regulates the net heating in the thermosphere. An increased eddy diffusion reproduces the effect of a possible auroral upwelling of methane but gives rise to an H2 temperature smaller than the observed value.

Ionospheric storm of early November 1993 in the Indian equatorial region

An investigation is made of the response of equatorial ionosphere in the Indian (75°E) sector to the major magnetic storm of November 3, 1993, using data from the ionosonde and magnetometer networks spanning the region 0.3–34.5°N dip. Some outstanding and new aspects of the storm time ionospheric behaviour are revealed. An anomalous and striking positive gradient in ƒ0F2 from the magnetic equator to 34.5° dip developed under counter electrojet (CEJ) condition in the morning on November 4, corresponding to the early stage of the storm main phase. This storm effect is attributed to plasma transport by a poleward surge in transequatorial winds due to large scale atmospheric gravity waves (AGWs) launched by auroral heating. Remarkable wave‐like variations in hpF2 and ƒ0F2 immediately followed at locations away from dip equator till local sunset, with concomitant disruptions in the development of the equatorial ionization anomaly (EIA). Rapid variations in meridional neutral winds due to large‐scale AGWs are assessed as the cause of the oscillations in hpF2 and the associated cyclic sequence of development and inhibition of EIA as the outcome of the combined effects of plasma transport due to meridional winds and the plasma “fountain” driven by E×B drift. Just after the onset of the storm recovery phase at 1800 LT, a sudden and anomalous drop (54–57%) in ƒ0F2 prevailed throughout the anomaly region over the interval 1915–2330 LT, with an apparent time delay in occurrence toward the magnetic equator. This premidnight collapse of equatorial F region is interpreted in terms of horizontal transport of plasma across the equator toward the opposite hemisphere by an equatorward surge in meridional winds. The case study showed that besides disturbances in the zonal electric field due to prompt penetration and ionospheric disturbance dynamo effects, perturbations in neutral meridional winds played a prominent role in the ionospheric storm of November 4, 1993, in the Indian equatorial region.

Dynamical effects of geomagnetic storms and substorms in the middle‐latitude ionosphere: An observational campaign

An observational campaign was conducted in October 1992 for ∼36 hours, at three high‐ to low‐latitude sites near 75°W longitude (Sondre Stromfjord, Millstone Hill, and Arecibo). Vector plasma drift velocities are obtained using the incoherent scatter radar technique at each site. Neutral winds were measured using a Fabry‐Perot interferometer, and 6300 Å airglow structures were imaged at the midlatitude site. Electric fields and meridional winds for the period were perturbed when magnetic storms and substorms occurred on the day and night of the campaign. The penetration of magnetospheric electric field and the following interplays between ionospheric electrodynamics and thermospheric wind perturbations in the midlatitude ionosphere are assessed using the multidiagnostic measurements. Evidence for traveling atmospheric disturbances (TADs) and large‐scale gravity waves induced by auroral heating effects upon the thermosphere is identified. Diffuse aurora and a stable aurora red (SAR) arc were observed from Millstone Hill during the night of the campaign. The SAR arc moved southward when there were westward electric field perturbations, indicating plasmasphere compression in the postmidnight sector under substorm conditions. The SAR arc location was used to infer the motion of the magnetospheric shielding layer past the Millstone Hill site. Ionospheric F region disturbances in hmF2, NmF2, and total electron content were driven by the observed dynamics, exhibiting a complex mix of wind and electric field perturbations. While standard model episodes of penetration and shielding/overshielding occurred during the daytime event, such unambiguous clarifications were far less obvious during the nighttime event. This is perhaps due to the prolonged period of moderate geomagnetic activity that served as the background conditions for the substorms that occurred during the campaign.

Supersonic winds in Jupiter's aurorae

Jupiter has a giant magnetosphere that is coupled to the planet's upper atmosphere; as the planet rotates, its magnetic field drags a dense ionized equatorial sheet of plasma, which must interact with the upper atmosphere. Jupiter's aurorae are much more powerful1,2 than the Earth's, and cause significant local heating of the upper atmosphere. Auroral electrojets—ion winds that race around Jupiter's auroral ovals—play a key role in theoretical models of how Jupiter's rotational energy is transferred to the plasma sheet3,4 and how winds may transport energy from auroral heating to lower latitudes5,6,7. But there has hitherto been no direct observational evidence for the existence of such electrojets. Here we report observations of electrojets that have winds approaching or in excess of the local speed of sound. The energy produced by these electrojets could heat the whole upper atmosphere, if the auroral regions couple efficiently with the rest of the planet.