Northern Auroral Region Research Articles

Velocity of small-scale auroral ionospheric current systems over Indian Antarctic station Maitri

The Indian Antarctic station Maitri (geog. 70‡45’S, 11‡45’E, geom. 66‡.03S, 53‡.21E) occupies a sub-auroral location during magnetically quiet conditions(Σκρ < 10), but attains an auroral position when the auroral oval shifts equatorwards with increasing strength of magnetic disturbance. At the latter times, triangulation with 3 fluxgate magnetometers located at the vertices of a suitable triangle provides a means of monitoring mobile auroral ionospheric current systems over Maitri. The spacing between the magnetometers is typically kept at 75–200 km, keeping in mind the scale-sizes of ∼ 100 km for these mobile current systems. This work reports the results of two triangulation experiments carried out around Maitri in January 1992 and January 1995, both during Antarctic summer. The velocities estimated for pulsations of the Pc4 and Pc5 type were about 0.59 km/sec in the direction 102‡.7 east of due north, in the first case, and about 1–3 km/sec in the second case in the east-west direction.While several magnetometer arrays exist in the northern auroral regions (e.g., the Alberta array in Canada, the Alaskan array in the U.S. and the IMS Scandinavian array), there is no report in literature of triangulation through arrays in Antarctica, except for a one-day study by Neudegget al 1995 for ULF pulsations of the Pc1 and Pc2 type. The velocities obtained for the Pi3 type of irregular pulsations over Antarctica in the present study tally well with those obtained for northern auroral locations.

Detection of Rapidly Varying H 2 Emissions in Jupiter's Aurora from the Galileo Orbiter

H 2 band emissions in Jupiter's northern auroral region at 1240 Å were highly variable on time scales of a few seconds in Galileo ultraviolet spectrometer (UVS) recorded data from November 5, 1996 (7:25:29 to 7:34:30 spacecraft event time (SCET)) during orbit Callisto-3 (C3). Emission at 1611 Å varied in phase with 1240 Å emission, but was much weaker. The 1240 and 1611 Å variability are evidence that the precipitating particles have a highly variable flux. The estimated 1611 Å/1240 Å photon flux ratio of 1.82±0.07 (after background subtraction) is consistent with auroral penetration of a slant column depth in CH 4 of (3.5±0.2)×10 16 cm −2. UVS observed both the main auroral arc and the polar cap region on the day side. The slit-averaged brightness at 1240 and 1611 Å increased by almost a factor of 2 on two occasions with durations (full-width at half-maximum) of order 100 s. A similar observation of the south polar aurora near the dawn terminator during Galileo orbit Ganymede-8 on May 6, 1997 (13:16:32 to 13:32:42 SCET) found both the 1240 and 1611 Å emissions to be more steady and gradually increasing. The variable 1240 and 1611 Å emissions seen on orbit C3 may be similar to variable polar cap “flares” reported in recent Hubble Space Telescope (HST) ultraviolet images of Jupiter from the new Space Telescope imaging spectrograph.

The northern auroral region as observed in nitric oxide

The distribution of thermospheric nitric oxide in the northern polar region has been measured from the Student Nitric Oxide Explorer (SNOE). A polar image of a one‐year average at 106 km shows the maximum density lies between 60° and 70° N geomagnetic latitude. The density around this auroral oval varies as a function of longitude with the larger densities occurring at western geomagnetic longitudes. A polar image of a second year of observations demonstrates the same latitudinal‐longitudinal density distribution indicating that the asymmetry is a persistent property of the auroral region. For a period of high geomagnetic activity, the nitric oxide observations indicate that the flux of precipitating electrons at 90° W geomagnetic longitude was 75% larger than the flux at 90° E geomagnetic longitude. We suggest that this asymmetry is caused by the longitudinal variation in the magnetic field strength around the auroral oval.

The Analysis of the H Lyman α Emission Line Profile from Jupiter's Aurora

We present the analysis and modeling of the emission spectra of the jovian northern auroral region taken from May 28 to June 3, 1993, with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. They extend from 1204 to 1240 Â covering H Lyman α and part of the Werner and the Lyman bands of H 2. We used the 2×2 arcsec large science aperture combined with the G160M grating (spectral resolution of 570 mÂ) centred on Jupiter's central meridian. The auroral region studied extends from 50 to 60° north latitudes and from 130 to 220° System III longitudes. Within the 1 arcsec pointing uncertainty, most of the region delineated by the theoretical ovals at 5.9 and 30 jovian radii (R J) in the VIP4 model of J. E. C. Connerney, M. H. Acuna, N. F. Ness, and T. Satoh (1998, New models of Jupiter's magnetic field constrained by the Io flux tube footprint. J. Geophys. Res. 103, 11,929–11,939), including the auroral oval derived from the Wide Field Planetary Camera 2 images by J. T. Clarke, G. E. Ballester, J. T. Trauger, R. Evans, J. E. P. Connerney, K. Stapelfeld, D. Crisp, P. D. Feldman, C. J. Burrows, S. Casertano, J. S. Gallagher, R. E. Griffiths, J. J. Hester, J. G. Hoessel, J. A. Holtzman, J. E. Krist, V. Meadows, J. R. Mould, P. A. Scowen, A. M. Watson, and J. A. Westphal (1996, Far-ultraviolet imaging of Jupiter's aurora and the Io “footprint”. Science 274, 404–409), was sampled. We derive auroral brightnesses (averaged over the slit) ranging from 20 kR in the 190–220° longitude region to 30 kR in the 130–160° longitude range. We use the theoretical model developed by D. Rego, R. Prangé, and L. Ben Jaffel (1999, Auroral Lyman α and H 2 bands from the giant planets. 3. Lyman α intensity and spectral profile including radiative effects and H 2 color ratios. J. Geophys. Res. Planets. 104, 5939–5954), which calculates self-consistently the auroral Lyman α line profile for electron and proton precipitations and a given frequency redistribution function. This model shows that Lyman α profiles are not dependent upon the identity of the particles for a given penetration depth. These profiles only constrain the atmospheric H column density above the emitting layer, H col. Best agreement with the data is found with the complete frequency redistribution (CR). We derive an auroral H col of 1.3×10 16 cm −2 for all spectra with an upper limit of 5×10 16 cm −2. Precipitating protons also produce fast H atoms by charge exchange which can also be excited, but the resulting Doppler-shifted profile was not detected. Present data thus rule out protons as being the only precipitating particles. In the auroral zones, theoretical models predict that chemical reactions induced by particle precipitations often result in the production of atomic hydrogen, and thus an enhancement of the H density is expected. However, the value of H col we derive, between ∼8 and 23 times less than the equatorial values of L. Ben Jaffel, J. T. Clarke, R. Prangé, R. Gladstone, and A. Vidal-Madjar (1993, The Lyman alpha bulge of Jupiter: Effects of a non-thermal velocity field. J. Geophys. Res. Lett. 20(8), 747–750), is in good agreement with the auroral values of R. Prangé, D. Rego, L. Pallier, L. Ben Jaffel, C. Emerich, J. T. Clarke, G. E. Ballester, and J. Ajello (1997, Detection of self-reversed Lyman α lines from the jovian aurorae with the Hubble space Telescope, Astrophys. J. 484, L169–L173). This suggests that the H abundance predicted with standard atmospheric model is unrealistically large in the auroral region.

Dayside/nightside auroral X ray emission differences‐Implications for ionospheric conductance

In this study we examine the occurrence probability of intense X ray emissions (&gt;2 keV) from the auroral ionosphere in daylight and in darkness. The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS Polar spacecraft remotely images the Earth's ionosphere from high altitude (apogee of about 9 Earth radii). This instrument is capable of observing the entire northern auroral region at once, and is normally unaffected by scattered sunlight; resulting in an unbiased study of differences in auroral emissions between daylight and darkness. The differences in the occurrence probability between daylight and darkness are interpreted as being due to differences in the local ionospheric conductivity, as described in previous studies using in situ particle populations and lower energy ionospheric emissions. There is also a clear dependence upon season which is in accordance with the Russell‐McPherron effect. Under similar geomagnetic conditions, comparisons of occurrence probability separated by whether each region is in daylight or in darkness illustrate that intense auroral emissions are consistently more likely when a given region is in darkness than when the same region is in daylight.

Lessons from the ring current injection during the September 24, 25, 1998 storm

Because the conditions in the solar wind that lead to the injection of energetic particles into the ring current simultaneously affect a variety of geomagnetic phenomena, it is often difficult to determine whether these magnetospheric processes are causally related or are simply contemporaneous. The sequence of interplanetary conditions leading up to the September 24–25, 1998 geomagnetic storm are particularly suited to illustrating what conditions lead to significant activity in the auroral zone as distinct from the injection into the ring current. This interval illustrates that the magnetosphere requires a period of strong, steady, not time‐varying, convection for the buildup of the ring current, and that the AE index cannot be used to forecast the ring current buildup. Furthermore, when the ring current reached its peak strength, it contained 6 PJ and was dissipating energy at a rate of about 0.4 TW. The northern auroral regions and southern added another 1.2 TW of dissipation. This rate of dissipation could be maintained readily by the mechanical energy flux of the solar wind of about 40 TW but would require coupling to much of the incident solar‐wind Poynting flux if that were the sole energy source.

Statistical X ray auroral emissions – PIXIE observations

The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS POLAR spacecraft has been making observations of X rays emitted simultaneously over the entire auroral zone since March, 1996. PIXIE remotely senses the bremsstrahlung produced by the loss of multi‐keV electrons from the magnetosphere into the ionosphere. In this study we examine twenty‐six months (April 1996 – July 1998) of X ray observations of the northern auroral region from the PIXIE instrument. X ray flux is obtained from the PIXIE observations, which allows us to produce for the first time synoptic auroral maps representing the precipitation of energetic electrons, as a function of geomagnetic activity.

Imaging Jupiter's Aurora at Visible Wavelengths

On November 9, 1996 and again on April 2, 1997, the Galileo spacecraft's Solid State Imaging (SSI) camera targeted the northern auroral region of Jupiter. These observations represent (i) the first spatially resolved images of the jovian auroral oval either at visible wavelengths or on the nightside of the planet, (ii) the first image at visible wavelengths of an auroral footprint of the Io Flux Tube (IFT), (iii) the first unambiguous detection at visible wavelengths of auroral emission on the jovian limb, and (iv) the first images of the aurora with spatial resolution below 100 km per pixel (46 and 35 km, respectively). Relative to many prior expectations, the visible aurora is (i) lower in altitude, (ii) associated with magnetic field lines that cross the equator closer to the planet, and (iii) more variable in time and space. The 1996 images used a clear (broadband) filter, while the 1997 images used both the clear filter and five narrower filters over wavelengths ranging from violet to 968 nm. The filtered images imply that the visible auroral emission contains atomic hydrogen lines, although there is also a continuum component. We were able to position the aurora in three-dimensional space and found the limb emission to be ∼240 km above the surface of a standard (P≈ 1 bar) reference ellipsoid. Our most accurate analysis of the equatormost part of the oval placed it at 54.5° planetocentric latitude and 168° west longitude. Combined with the latest magnetic field models, our results imply that the particles that cause the aurora originate in Jupiter's equatorial plane ∼13RJfrom the center of the planet. The oval was brighter and wider in the 1996 images than in the 1997 images. The broadband radiance of a typical place on the oval as seen directly overhead varied from ∼80 kR in 1997 to ∼300 kR in 1996. Our estimates of the full width of the oval varied from under 500 km to over 8000 km, partly depending on the signal-to-noise ratio of the image. The radiated power per unit length along the oval ranged from ∼60 to ∼700 W/m, with the associated radiated power from the entire oval varying from ∼109to ∼9 × 1010W. Appreciable auroral emission also occurred both north and south of the main oval. One image contains the northern footprint of the IFT, which appears as a central ellipse with a tail of emission that lies downstream with respect to the plasma flow past Io. The central ellipse is ∼1200 km downstream by ∼500 km cross stream. The IFT is comparable in brightness to the nearby auroral oval (∼250 kR) and has a total radiated power of ∼3 × 108W.

Thermal profiles in the auroral regions of Jupiter

The temperature structure within the northern auroral region of Jupiter is studied by reanalyzing the Voyager 1/infrared interferometer and radiometer spectrometer (IRIS) spectra. The total measured excess infrared auroral zone emission (averaged over the IRIS field of view) in the hydrocarbon bands between 7 and 13 μm is found to be about 208 ergs cm−2 s−1 over an area of about 2 × 1018 cm2 with a resulting power output of 4 × 1013 W. In comparison, the total energy deposition by magnetospheric charged particles has been estimated on the basis of UV observations to range between 1 × 1013 and 4 × 1013 W over a comparable area. The large amount of radiated energy observed in the infrared may imply an additional heat source in the auroral regions (possibly Joule heating). A new set of thermal profiles of Jupiter's high‐latitude upper atmosphere has also been derived. These profiles have a large temperature enhancement in the upper stratosphere and are constrained to reproduce the CH4 emission at 7.7 μm. The emission in the other hydrocarbon bands (C2H2 and C2H6) is found to depend on the depth to which the temperature enhancement extends, which further constrains the thermal profiles. This study shows that a large temperature enhancement in the upper stratosphere and lower thermosphere can explain the observed excess hydrocarbon emission bands; thus smaller variations in hydrocarbon abundances (between the high latitudes and the equatorial and middle latitudes) are required than has been assumed in previous models.

Spatial and Spectral Characteristics of the Near-Infrared Aurorae of Jupiter

We obtained near-infrared images and spectra of the Jovian aurorae using the CRSP (Cryogenic Spectrometer) on the KPNO 2.1-m telescope on January 22, 1992 (UT). The slit of the CRSP was aligned with 63° north and south latitudes in order to study the spatial structure of the auroral emission between 2 and 4μm. The spatial resolution was about 3 arcsec with spectral resolving powers of about 1000 and 700 for 2 and 4 μm, respectively. We obtained spatially resolved emission intensities of (1) H + 3 fundamental band lines between 3 and 4 μm and (2) the H 2 quadrupole lines of Q 1(1), Q 1(3), and S 1(1), and several H + 3 lines between 2 and 2.5 μm in the northern auroral region. The northern bright spot as seen in the 2-μm H 2 and H + 3 emission lines is at nearly the same position as the northern bright spot due to the H + 3 fundamental band lines. We derived spatially resolved rotational temperatures from the H + 3 lines in the north polar region and found that temperatures on the bright spot are no greater than those in other auroral regions. This suggests an enhanced abundance of H + 3 on the bright spot relative to other parts of the auroral zone. We found the southern H + 3 line intensities to be much lower than those in the northern auroral zone, suggesting a lower level of auroral activity near the south pole at the time of the observations. The continuum near the known H + 3 lines in the 3.17- to 3.57-μm region shows a significant brightening around the northern bright spot position, implying that the continuum contains many weak unidentified emissions, which are correlated with the auroral activity.

Infrared studies of hydrocarbons on Jupiter

Measurements of ethane and ethylene emission lines on Jupiter were made using a dual CO 2 laser heterodyne spectrometer. Retrieved abundances of these products of methane photolysis in the Jovian stratosphere were used to test existing photochemical models and to investigate a localized “hot spot” in the northern auroral region.

Atmospheric density and rotation below 195 km from a high resolution drag analysis of the satellite OV1-15 (1968-059A)

A drag analysis using high-accuracy C-band radar-transponder data has yielded densities at heights between 135 and 195 km at approximately 1-day intervals over a 4-month period in 1968. On the average, the results were in reasonable agreement with currently used model atmospheres, but the variability of the density was found to be much larger than was predicted by the models. Density variations of the order of 10%–20% at 170 km were observed to correlate with moderate geomagnetic activity and the semiannual effect. A large increase of about 35% at 140 km was observed during a major geomagnetic storm. Increases in the density at times of geomagnetic activity were significantly larger in the northern auroral region than elsewhere. The effects of solar activity, the diurnal bulge, and latitude were not apparent in the data. The atmospheric rotation rate at 175 km was found to be higher than that of the earth by a factor of about 1.3.

Relations between the sporadicE-layer and geomagnetic disturbances in the northern auroral and polar regions

Unter Anwendung der auf 12 Uhr GMT bezogenen Beobachtungen aus den 33 ionospharischen und geomagnetischen Stationen wurde eine statistische Analyse der Beziehungen zwischen dem Vorkommen der sporadischen E-Schicht und den geomagnetischen Storungen durchgefuhrt. Es wurde keine einfache oder eindeutige Abhangigkeit gefunden. Die Ergebnisse deuten nur an, dass man grosseres Vorkommen der sporadischen E-Schicht in magnetisch mehr gestorten Zeitabschnitten erwarten kann.

A survey of low-energy (E≳5 KeV) electron energy fluxes over the northern auroral regions with satellite Injun 4

Observations of the energy fluxes of ≳5-kev electrons at low altitudes (∼525–2500 km) over the northern auroral regions have been made by the nearly polar orbiting Injun 4 satellite, from February 1965 through July 1966, during a period of minimum solar activity. Results of the first detailed survey of the percentages of occurrence of the energy fluxes of ≳5-kev electrons (with pitch angles ∼40°±15°) in the outer radiation/auroral zone, as a function of magnetic local time and invariant latitude, are graphically displayed for various energy flux thresholds. The greatest occurrences of these energy fluxes were observed in the night-dawn-morning hours of local time, at latitudes from ∼65° to ∼70°, with median energy fluxes as great as 0.1 ergs(cm² sec sr)−1. The spatial distributions of these contours are compared with those for the occurrences of optical auroras. Limited simultaneous observations of these energy fluxes at pitch angles of ∼40° and ∼90° have shown that the ratios of these energy fluxes are typically greater than 0.1, with a condition of isotropy occurring for energy fluxes ≳1 erg (cm² sec sr)−1.

Simultaneous Geomagnetic and Cosmic Noise Absorption Pulsations near Geomagnetic Noon

SEVERAL groups1,2 using balloon-borne instruments in the northern auroral region have observed pulsating electron precipitation events occurring near local noon which were associated with geomagnetic pulsations of type Pc3 (continuous pulsations with periods in the range 10–45 s). Pulsation events which are believed to be of the same basic type have been recorded with ground-based instruments, namely a riometer and geomagnetic micropulsations (dH/dt) sensor, at Macquarie Island in the southern auroral region.

On the latitude variation of scintillations of ionosphere origin in satellite signals

A cooperating group of European and American observatories has made observations of scintillated 54 MHz signals from the satellite Transit 4A and of 40 MHz signals from the ionospheric beacon satellite, S-66 (BE-B). Analysis of these data shows that the statistical behaviour of the ionospheric irregularities causing scintillation can be represented by a smooth latitude function which displays increasing depth of scintillation near the northern auroral region. For mid-latitude stations, observations away from the zenith may be interpreted as a geometric modification of the smooth latitude dependence but the behavior of off zenith observations made near the auroral zone is presently speculative.