Time-sequence of polar magnetic substorms

The time-sequence of polar magnetic substorms is discussed to clarify some controversies on the magnetospheric substorm model including the growth phase. The main purpose of the analyses is to examine magnetic variations in the polar cap and in low latitudes. The onset of the expansion phase is confirmed to be reasonably defined by a vector change of polar-cap magnetic disturbance, a sharp intensification of the auroral electrojet disturbance and the beginning of positive ΔH disturbance in midlatitudes near midnight. It is shown that the growth phase signatures so far proposed are consistent when the onset of the expansion phase is identified from the above mentioned features.     

The sources of the polar cap and low latitude bay-like disturbances during substorms

Examination of the polar cap and low-latitude bays during substorms shows that there are two types of disturbances, DP₁₁ and DP₁₂, which are different not only in their morphological features, but in their origin as well. The DP₁₂ disturbances are associated with pure ionospheric currents, whereas the DP₁₁ are though to be generated by the Birkeland type current system. This conclusion is based on examination of the following characteristics: (1) the seasonal changes of the DP₁₁ and DP₁₂ disturbances in the polar cap, (2) the seasonal variations of the low-latitude bay intensity at the conjugate points in the cases of the DP₁₁ and DP₁₂ disturbances, (3) the distribution of the intensity of the DP₁₁ and DP₁₂ disturbances in both northern and southern hemispheres along the midnight meridian.     

Ordinary and Z-mode emissions from the Jovian polar region

The Ulysses Unified Radio and Plasma (URAP) experiment has detected a new component of Jupiter's radio spectrum in the frequency range from about 10 to 30 kHz. This component is emitted in the magnetoionic ordinary mode from a localized corotating source in the northern polar region. The source is centered at system III longitude 208°, near the meridian containing the North magnetic dipole axis, at a distance of nearly 4R_J from the planet and near the last closed field line. The emission frequency is somewhat above the electron plasma frequency in the source region, but well below the electron gyrofrequency. Accompanying this O-mode emission at lower frequencies is intense Z-mode emission, which is likely to play a significant role in the generation of the O-mode.     

Observations of dawn–dusk aligned polar cap aurora during the substorms of January 21, 2005

A new type of polar cap aurora, dawn–dusk aligned polar cap aurora (DDAPCA), was detected during the exceptionally intense January 21, 2005 substorm (AE_max=3504 nT). The DDAPCA was located at very high latitude (>85° MLAT) in the polar cap region. As the interplanetary magnetic field (IMF) GSM By component rotated from a positive to a negative value, the DDAPCA tilt angle relative to the dawn–dusk direction rotated anticlockwise and reached ∼45°. It is speculated that the DDAPCA arises from the formation of an X-line in the distant (>80R_E) tail due to polar cap magnetic field reconnection under unusually high solar wind compression conditions.     

Characteristics of optical emissions and particle precipitation in polar cap arcs

Simultaneous optical and particle data from the ISIS-2 satellite are used to characterize polar cap arcs. Polar cap arcs are identified from two-dimensional geomagnetic transforms of the optical data along with precipitating electron data for the time at which the satellite is on the field line intersecting the arc. No precipitating protons were detected for any of the arc crossings. The pitch angle. distribution of the precipitating electrons is generally isotropic and the differential electron spectra show enhancements in the flux in the 300–750 eV energy range. The average energy of the precipitating electrons for the different arcs ranges from about 300 to 600 eV. A possible explanation of the observed precipitating particle characteristics is that parallel electric fields are accelerating polar rain type spectra at an altitude of several thousand km. For the arc crossings reported here the equivalent 4278 Å emission rate per unit energy deposition rate has a mean value of 162 R/(erg cm^?2 s^?1). Average 3914 Å intensities are about 0.8 kR while 6300 Å intensities range from 0.5 to 3 kR. Model calculations indicate that direct impact excitation is a minor source for the 5577 Å emission rate, but supplies approx. 40% of the 6300 Å emission.     

Temporal and spatial variations in the Io torus

Spectrographic data on the Io torus from 15 nights of observations spread over a 4-month period in 1981 are presented here. The [SII] λλ6716, 6731; [SII] λλ4069, 4076; [OII] λλ3726, 3729; and [SIII] λ3722 lines were simultaneously measured on each spectrogram. An east-west asymmetry was observed in the optical emissions, showing larger western intensities and a more diffuse and radially extensive nebula to the east. Two configurations of [SII] longitudinal asymmetry that were stable over at least 4 days were observed. The magnetic longitudes of ～ 180 and 300° are shown to be of particular interest. Longitudinal structure was not detected in either the [OII] intensity or the plasma density as measured by the [OII] and [SII] doublet ratios. Errors in the line ratios could mask density variations as large as a factor of ～ 1.5. A radial variation in the ratio of OII/SII was observed, with the ratio being largest near Io's orbit. Monthly variabilitywas detected in both the intensity and density of the tours. The [SII] line ratios indicated an increase in n_e over the 4-month period that was accompanied by increased intensities. For single measurements, no correlation between the [SII] intensities and the [SII] λ6716/λ6731 line ratio was detected, but this could be a result of errors in the line ratio determinations. Extremely low values of this same ratio were measured. These appear to indicate errors in the presently accepted [SII] transition probabilities. These [SII] line ratios indicate that very-high-density regions are present in the torus, and it is shown how these regions could have significantly influenced these measurements.     

Temporal variations of polar cap 0I 5577-Å airglow

Limb observations of the E-region 0I 5577-Å airglow in the polar caps were obtained during 1971 and 1972 from the ISIS-2 satellite. In the southern polar cap the emission rate decreased to a broad minimum of 100 R near solstice, then increased later in the winter. In the northern polar cap a maximum was observed in November. In addition to large daily variations, there were enhancements that lasted several weeks. From a correlation analysis using both satellite data supplemented by the more extensive data base of Mullen et al. (1977,Planet. Space Sci.25, 23) and stratospheric temperatures, it is clear that stratospheric warmings result in polar cap airglow enhancements. The most plausible cause of these airglow enhancements appears to be increased meridional transport of atomic oxygen into the polar cap.     

The spatial distribution of polar hydrogen deposits on the Moon

A new analysis of the Lunar Prospector epithermal neutron data is presented, providing an improved map of the distribution of hydrogen near to the lunar poles. This is achieved using a specially developed pixon image reconstruction algorithm to deconvolve the instrumental response of the Lunar Prospector's neutron spectrometer from the observed data, while simultaneously suppressing the statistical noise. The results show that these data alone require the hydrogen to be concentrated into the cold traps at up to 1 wt% water-equivalent hydrogen. This combination of localisation and high concentration suggests that the hydrogen is present either in the form of a volatile compound or as solar wind protons implanted into small regolith grains.     

Observed characteristics of polar cleft Hα and OI emissions

Observations at Godhavn, Greenland show that the intersection of the polar cleft with the ionosphere can be recognized by simultaneous occurrence of hydrogen emissions (H_α, H_β>) and enhanced OI 6300 Å emission. The H_α-line reveals a characteristic narrow and symmetric Doppler profile which is interpreted as indicating that the solar wind protons retain their typical flux and energy spectrum all the way down to the ionosphere. The cleft intersection seems to cover the sector 04:00–22:00 geomagnetic time.     

Dynamics of day and night aurora during substorms

The motion of auroral forms on the day- and nightside of the Earth has been studied during different substorm phases by means of all-sky camera films. A substorm is characterized by a shift of the luminescence region towards the equator at noon and mainly towards the pole at midnight. However, individual forms drift predominantly toward the pole on the dayside and towards the equator on the nightside. The velocity of the poleward motion at noon is largest during the expansive phase of a substorm and amounts on the average to 330 $\text{m}\text{sec}$ but even during relatively quiet magnetic conditions a poleward motion is observed.     

Origin of Jovian decameter wave emissions— Conversion from the electron cylotron plasma wave to the ordinary mode electromagnetic wave

Energy conversion rates from the extraordinary mode to the ordinary mode ofthe electromagnetic waves in the Jovian plasmasphere has been calculated for a model of the sharp boundary that is given in the vicinity of the position where ω = ω_p, for an angular frequency ω and the angular plasma frequency ω_p. The extraordinary mode electromagnetic wave that is obtained as a result of the transformation of a longitudinal propa- gating through an inhomogenous plasma is here considered. The results give conversion rates of 1–50 per cent, at the most, when a wave normal direction of an is nearly parallel to the boundary normal direction and when the Jovian magnetic field vector is close to the boundary normal direction within an angle range from 10° to 15°. The electric field intensity, in range from 7 to 70 mV/m, of the original electrostatic electron cyclotron plasma waves can give the power flux in a range from 10^-22 to 10^-20W/m² Hz for the Jovian decameter waves observed at the Earth's surface. Efficient energy conversion is possible only when the ray direction of the emitted wave is in nearly perpendicular direction with respect to the magnetic field; this is the origin of the sharp beam emission of the Jovian decameter wave burst.     

The morphology of n and no in auroral substorms

The effects of a typical auroral electron precipitation substorm sequence on odd nitrogen species in the thermosphere have been investigated. The analysis makes use of the time dependent model of the aurora developed by Roble and Rees (1977), which couples the thermal properties to the ionospheric chemistry and transport self-consistently and includes diffusive transport of NO, N(²D) and N(⁴S). A substantial increase in the E-region density of NO or of N(⁴S) is predicted, with the result depending on the production ratio of N(²D) to N(⁴S) in the aurorally dominant source mechanism, electron impact dissociation of N₂. A production ratio that favors N(²D) by a factor of one half or larger leads to enhancement of NO, while a ratio of $\text{1}\text{4}\text{N}\text{(}{}^2\text{D}\text{)+}\text{3}\text{4}\text{N}\text{(}{}^4\text{S}\text{)}$ results in a buildup of N(⁴S). The cyclical behaviour of the substorm, i.e. alternate intervals of electron precipitation and quiet periods, accentuates the scavenging effect of the initially dominant odd nitrogen species upon the less abundant one.     

Dynamical modeling of a planetary wave mechanism for a Martian polar warming

A dynamical mechanism for the Martian (atmospheric) polar warming observed by the Viking IRTM during the 1977 winter solstice dust storm (and a similar one possibly observed by Mariner 9 in 1971) is proposed, and investigated using a simplified nonlinear model. The model is of a type previously used to successfully simulate the essential aspects of terrestrial sudden stratospheric warmings. The dynamical mechanism is, in part, very similar fundamentally to that responsible for these warmings, involving planetary-scale waves. A number of numerical experiments have been conducted to assess the basic viability of such a mechanism for the martian polar warming and to examine its sensitivity to several factors. These experiments demonstrate that a planetary wave mechanism can produce a polar warming with the magnitude and suddenness of that observed. A planetary wave mechanism must primarily involve wavenumber 1, as wavenumber 2 is too strongly vertically trapped to produce a warming like that observed. The necessary wave forcing in the present model can be topographic (mechanical) or thermal (and nonstationary), and is relatively large but certainly plausible. The strong radiative damping in the Mars atmosphere acts to substantially inhibit a warming, through its effects on both the zonal flow and the wave. Dissipation plays a greater role relative to transience in a model Martian warming of the type studied here than in a sudden stratospheric warming. Increasing radiative damping during a warming due to higher temperatures and greater dust loading may play a role in yielding a relatively rapid cooling phase for the Mars warming event. The residual mean meridional circulation during a model warming entails strong poleward and downward flow into high northern latitudes, throughout a very deep region. This probably indicates similar transport of atmospheric dust, as well as water. Such transports are of considerable potential significance for both the dust and water cycles on Mars.     

Temporal and spatial distribution of GPS-TEC anomalies prior to the strong earthquakes

Earthquakes are one of the most destructive and harmful natural disasters, especially in recent years, the 2008/5/12 Wenchuan M7.9 earthquake, the 2011/3/11 Tohoku M9.0 earthquake and the 2012/4/11 Sumatra M8.6 earthquake have caused a significant impact to the human life. In this paper, we make a study of the temporal and spatial distribution of the Global Positioning System Total Electron Content (GPS TEC) anomalies prior to the three strong earthquakes by the method of statistical analysis. Our results show that the pre-earthquake ionospheric anomalies are mainly positive anomalies and take the shape of a double-crest structure with a trough near the epicenter. The ionospheric anomalies do not coincide with the vertical projection of the epicenter of the subsequent earthquake, but mainly localize in the near-epicenter region and corresponding ionospheric anomalies are also simultaneously observed in the magnetic conjugate region prior to the three earthquakes. In addition, the amplitude and scale-size of the ionospheric ΔTEC are different with the magnitude of the earthquake, and the horizontal scale-size of the greatest anomalies before the Tohoku M9.0 earthquake is ～30^° in longitude and ～10^° in latitude, with the maximum amplitude of TEC disturbances reaching ～20 TECu relative to the background. The peak of anomaly enhancement usually occurs in the afternoon to sunset (i.e. between 14:00 and 18:00 local time) which lasts for approximate 2 hours. Possible causes of these anomalies are discussed, and after eliminating the effect of solar activities and magnetic storms it can be concluded that the detected obvious and regular anomalous behavior in TEC within just a few days before the earthquakes is related with the forthcoming earthquakes with high probability.     

South polar residual cap of Mars: Features, stratigraphy, and changes

The south residual polar cap of Mars, rich in CO₂ ice, is compositionally distinct from the north residual cap which is dominantly H₂O ice. The south cap is also morphologically distinct, displaying a bewildering variety of depressions formed in thin layered deposits, which have been observed to change by scarp retreat over an interval of one Mars year (Malin et al., 2001, Science 294, 2146-2148). The climatically sensitive locale of the residual caps suggests that their behavior may help in the interpretation of recent fluctuations or repeatability of the Mars climate. We have used Mars Global Surveyor Mars Orbiter Camera (MOC) images obtained in three southern summers to map the variety of features in the south residual cap and to evaluate changes over two Mars years (Mars y). The images show that there are two distinct layered units which were deposited at different times separated by a period of degradation. The older unit, ∼10 m thick, has layers approximately 2 m thick. The younger unit has variable numbers of layers, each ∼1 m thick. The older unit is eroding by scarp retreat averaging 3.6 m/Mars y, a rate greater than the retreat of 2.2 m/Mars y observed for the younger unit. The rates of scarp retreat and sizes of the different types of depressions indicate that the history of the residual cap has been short periods of deposition interspersed with longer erosional periods. Erosion of the older unit probably occupied ∼100-150 Mars y. One layer may have been deposited after the Mariner 9 observations in 1972. Residual cap layers appear to differ from normal annual winter deposits by having a higher albedo and perhaps by having higher porosities. These properties might be produced by differences in the depositional meteorology that affect the fraction of high porosity snow included in the winter deposition.     

Effects of the interplanetary magnetic field on the magnetotail structure: Large-scale changes of the plasma sheet during magnetospheric substorms

The effects of the orientation of the interplanetary magnetic field (IMF) on the structure of the distant magnetotail are studied by superposing a uniform magnetic field on a magnetospheric model. It is shown that a southward component of the IMF alone can reduce the closed field region in the magnetotail, while a northward turning of the IMF can produce a new closed field region. It is suggested that these two effects can explain thinning and thickening, respectively, of the plasma sheet during magnetospheric substorms without invoking internal instabilities.     

Morphology and spatial distribution of XUV and X-ray emissions in an active region observed from Skylab

We studied the morphology and spatial distribution of loops in an active region, using coordinated observations obtained with both the S082A XUV spectroheliograph and the S056 grazingincidence X-ray telescope on Skylab. The active region loops in the temperature range 5 × 10⁵ –3 × 10⁶ K fall basically into two distinctive groups: the hot loops with temperatures 2–3 × 10⁶ K as observed in coronal lines and X-rays, and the relatively cool loops with temperature 5 × 10⁵ –1 × 10⁶ K as observed in transition-zone lines (Ne vii, Mg ix). The brightest hot coronal loops in the active region are mostly low-lying, compact, closely-packed, and show greater stability than the transition-zone loops, which are fewer in number, large, and slender. The observed aspect ratio of the hot coronal loops is in the range of 0.1 and 0.2, which are almost two orders of magnitude larger than those for the Ne vii loops. Brief discussion of the MHD stability of the loops in terms of the aspect ratio is presented.     

Temporal changes in the flux of meteorites in the recent past

The depth variations of the fossil cosmic ray tracks and agglutinates have been examined in the (0.6–0.7)m deep Apollo 12 and 16 drive cores, in the 2.4 m Apollo 15 deep drill core and in a 0.6 m long section of the Apollo 17 deep drill core. These data indicate Moon-wide short duration episodes of impacts of meteorites of size 10 cm^–1m on the lunar surface. Based on the longest continuous Apollo 15 deep drill core record, these impact episodes occurred about 150, 400 and 700 m.y. ago. The enhancements in the meteorite flux may be due to solar dynamical processes or they may be related to excursions of the solar system, once in each orbit, through a certain dusty region of the galaxy.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.     

Residual south polar cap of Mars: Stratigraphy, history, and implications of recent changes

The residual south polar cap (RSPC) of Mars includes a group of different depositional units of CO₂ ice undergoing a variety of erosional processes. Complete summer coverage of the RSPC by ∼6-m/pixel data of the Context Imager (CTX) on Mars Reconnaissance Orbiter (MRO) has allowed mapping and inventory of the units in the RSPC. Unit maps and estimated thicknesses indicate the total volume of the RSPC is currently <380 km³, and represents less than 3% of the total mass of the current Mars atmosphere. Scarp retreat rates in the CO₂ ice derived from comparison of High Resolution Imaging Science Experiment (HiRISE) data with earlier images are comparable to those obtained for periods up to 3 Mars years earlier. These rates, combined with sizes of depressions suggest that the oldest materials were deposited more than 125 Mars years ago. Most current erosion is by backwasting of scarps 1-12 m in height. This backwasting is initiated by a series of scarp-parallel fractures. In the older, thicker unit these fractures form about every Mars year; in thinner, younger materials they form less frequently. Some areas of the older, thicker unit are lost by downwasting rather than by the scarp retreat. A surprising finding from the HiRISE data is the scarcity of visible layering of RSPC materials, a result quite distinct from previous interpretations of layers in lower resolution images. Layers ∼0.1 m thick are exposed on the upper surfaces of some areas, but their timescale of deposition is not known. Late summer albedo changes mapped by the CTX images indicate local recycling of ice, although the amounts may be morphologically insignificant. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data show that the primary material of all the different forms of the RSPC is CO₂ ice with only small admixtures of water ice and dust.