J. B. Sigwarth

The Visible Imaging System (VIS) for the Polar Spacecraft

The Visible Imaging System (VIS) is a set of three low-light-level cameras to be flown on the POLAR spacecraft of the Global Geospace Science (GGS) program which is an element of the International Solar-Terrestrial Physics (ISTP) campaign. Two of these cameras share primary and some secondary optics and are designed to provide images of the nighttime auroral oval at visible wavelengths. A third camera is used to monitor the directions of the fields-of-view of these sensitive auroral cameras with respect to sunlit Earth. The auroral emissions of interest include those from N2+ at 391.4 nm, Oi at 557.7 and 630.0 nm, Hi at 656.3 nm, and Oii at 732.0 nm. The two auroral cameras have different spatial resolutions. These resolutions are about 10 and 20 km from a spacecraft altitude of 8Re. The time to acquire and telemeter a 256×256-pixel image is about 12 s. The primary scientific objectives of this imaging instrumentation, together with thein-situ observations from the ensemble of ISTP spacecraft, are (1) quantitative assessment of the dissipation of magnetospheric energy into the auroral ionosphere, (2) an instantaneous reference system for thein-situ measurements, (3) development of a substantial model for energy flow within the magnetosphere, (4) investigation of the topology of the magnetosphere, and (5) delineation of the responses of the magnetosphere to substorms and variable solar wind conditions.

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

[1] During 2001 and 2002, when the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE-FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (AMLT) is strongly correlated with the IMF clock angle (θ C ) and a linear fit, ΔMLT = -0.017c C + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

Auroral poleward boundary intensifications and tail bursty flows: A manifestation of a large‐scale ULF oscillation?

[1]xa0Auroral zone observations often show significant ULF power. We have analyzed auroral and plasma sheet observations during two prolonged periods of strongly southward and relatively steady interplanetary magnetic field (IMF). We find evidence that auroral poleward boundary intensifications (PBIs), which have large intensity and occur repetitively throughout such periods, may be a manifestation of a large-scale ULF oscillation mode that strongly perturbs the plasma sheet and the auroral ionosphere. If this is correct, then ULF modes would be a major component of tail dynamics, of magnetosphere coupling to the ionosphere, and of auroral zone disturbances during periods of enhanced convection. They would simultaneously affect a large region of the nightside, extending along auroral zone field lines from the ionosphere to the equatorial plasma sheet and extending from field lines that lie near the magnetic separatrix to, at times, as close to the Earth as synchronous orbit. They would also occasionally have amplitudes as large as the changes that occur in association with other auroral zone disturbances such as substorms. Here we have found peak-to-peak amplitudes as high as several hundred nanoteslas in ground X, an order of magnitude in synchronous energetic proton fluxes, ∼20–40 nT in synchronous magnetic field components, ∼20 nT in tail magnetic field components, ∼1000 km/s in tail flow speeds, and ∼400 m/s in ionospheric flow speed. We find evidence for significant power at 0.5–0.7 mHz (∼25–30 min period), significant power at a possible second harmonic (∼1.1–1.3 mHz), and power at frequencies that could be higher harmonics simultaneously within the auroral ionosphere and within the nightside plasma sheet.

Does the braking of the fast plasma flow trigger a substorm?: A study of the August 14, 1996, event

[1]xa0In a substorm event of August 14, 1996, the Geotail satellite was at XGSM ∼ −10 RE aligned with the GOES 8 satellite in the X direction in the premidnight sector. Two Pi2 onsets were identified. Whereas the first onset was associated with a pseudobreakup, an auroral bulge developed following the second onset. At the pseudobreakup Geotail observed a fast (>900 km/s) plasma flow along with dipolarization, but the geosynchronous magnetic field became stretched subsequently. In contrast, for the main onset, the magnetic field dipolarized at GOES 8, although the associated total (time-integrated) transport of magnetic flux at Geotail was no stronger than that for the pseudobreakup. It is suggested that the braking of the fast flow does not necessarily cause tail current disruption in a large scale, and there is an additional (or alternative) condition to suffice for an initial brightening to develop to a global substorm.

Simultaneous images of the northern and southern auroras from the Polar spacecraft: An auroral substorm

[1]xa0The first simultaneous images of the auroras in the Northern and Southern Hemispheres with a single camera have been gained with the Earth Camera onboard the Polar spacecraft with sufficient angular resolution to provide maps of the intensities. An auroral substorm was fortuitously observed on 1 November 2001 when the spacecraft was favorably positioned to observe both auroras. The auroral intensities during the onset and the early expansive phase were mapped into corrected geomagnetic coordinates. The onset brightening at far-ultraviolet wavelengths was first detected in the southern auroras and subsequently seen in the northern auroras about 1 min later on L-shells of about 5 RE. During the early phases of the auroral expansive phase, the southern aurora continued to be brighter. The unequal auroral intensities in the two hemispheres suggest that there is strong coupling of the ionosphere and magnetosphere. The mapping of the auroras revealed that there is a local time difference between the onset positions of about 40 min, with the southern auroras positioned to the east relative to those in the north. This displacement was also similarly detected in the quiet auroral regions, which were not participants in the auroral substorm. Such displacements may be useful in developing improvements in the accuracy of global magnetic models that are important for determination of the position of the in situ particle and field measurements in the magnetosphere. The greater brightness of the northern quiet time auroras by tens of percent as first reported by aircraft observations at visible wavelengths in both hemispheres has been verified by the more global view from the Polar spacecraft.

Plasma Waves Observed During CUSP Energetic Particle Events and Their Correlation With Polar and Akebono Satellite and Ground Data

We present Polar plasma wave data during cusp energetic particle (CEP) events at 6–9 RE. These data suggest the presence of coherent electrostatic structures that are highly localized and that have typical velocities on the order of hundreds to thousands of km/s along the ambient magnetic field. Some of the wave signatures are solitary waves and some are wave packets. The Polar wave instrument also provides evidence that some of the bursts of electromagnetic waves (with frequencies of a few hundred Hz and just below the electron cyclotron frequency around 800 Hz to 1–2 kHz) that are observed are cohenrent and propagating both up and down the field lines. Electron cyclotron harmonic (ECH) waves are oftern detected but their duration is usually short (< 1 s). Low Frequency (<1 kHz), broadband, bursty electromagnetic waves are also present. The Polar wave data results are used to obtain a better understanding of the macro/microphysics during a CEP event that takes place on September 11, 1996, by correlating various Polar (∼ 7.0 RE) and Akebono (∼ 1.4 RE) data while both spacecraft are in or near the cusp/cleft region and nearly on the same field line, and magnetometer data from the Canadian Intermagnet and Canopus ground stations, which lie near the base of the magnetic pootprint passing through Polar. Solar wind and magnetic field data from the interplanetary medium and magnetosheath are provided by the Geotail and IMP-8 satellites, respectively. Some of the cusp waves may be indicators of the reconnection process taking place through the cusp, the result of mixing of magnetosheath with magnetospheric plasma, and the consequence of an anisotropic electron population in a depressed magnetic field. The low frequency electromagnetic waves are still under study to determine their role, if any, in the heating and acceleration of the MeV He ions during CEP events.

Auroral electrojet configuration during substorm growth phase

[1]xa0The spatial configuration of the auroral electrojets during the growth phase of classical substorms is investigated. Electrojet intensities are determined from measurements of the convection electric field and calculated height integrated conductivity. Both the westward and eastward electrojets decrease in latitudinal width towards ∼21–23 MLT where they both are terminated. The latitudinal widths of the eastward electrojet as a function of MLT, however, show significant scatter. This scatter is shown to be effectively minimized by organizing the data by the local time distance to the future optical onset location rather than using magnetic local time. We find that the future optical onset will be located in the region of overlapping eastward and westward electrojets. The fact that the future onset location better organizes the data suggests that during the growth phase the magnetosphere is organizing itself for an onset at an already determined MLT as seen at ionospheric altitudes. Neither the IMF By nor IMF clockangle provide a simple explanation of the variation in the onset location.

Simultaneous observations of the auroral ovals in both hemispheres under varying conditions

[1]xa0This is the first analysis to use simultaneous observations of the entire auroral ovals in both hemispheres to track their location. Data was used from the Polar VIS and IMAGE FUV imagers on 23 October 2002 and plotted in AACGM coordinates. Results showed the expected IMF By-dependent asymmetry along the dawn-dusk meridian; however, there was an unexpected offset of both ovals toward dawn. Evidence is also shown for an asymmetry along the noon-midnight meridian dependent on both dipole tilt angle and the sense of IMF Bx. During a brief period of weak IMF Bz > 0 and IMF Bx > 0, the southern oval is observed to move equatorward relative to the northern oval, consistent with tail lobe reconnection occurring only in the southern hemisphere. This has important implications for the global response of the magnetosphere to different interplanetary conditions.

Responses of the open-closed field line boundary in the evening sector to IMF changes: A source mechanism for Sun-aligned arcs

[1]xa0Simultaneous measurements from the Polar satellite and by ground-based optical sensors suggest that brief variations of the poleward auroral boundary on the nightside correlate with changes in the interplanetary magnetic field (IMF) about an hour after a structure has propagated in the solar wind past the Earth. Short-lived Sun-aligned arcs may emerge along the open–closed magnetic field line boundary (OCB) and then disappear after ∼10 min. The arcs are fueled by energetic particles whose spectral characteristics are similar to those of the of boundary plasma sheet (BPS). Polar measurements confirm that these auroral protrusions into the polar cap occur on nearly isolated closed magnetic flux. Optical emissions from these arcs appear strongest at their intersection with the poleward boundary of the auroral oval. Detailed magnetohydrodynamic (MHD) simulations of dayside interactions, when the dominant IMF component BY changes sign, indicate that near the polarity reversal merging can occur between interplanetary field line segments within the magnetosheath [Maynard et al., 2001b]. Newly formed loops of interplanetary flux sweep past the Earth without interacting with the magnetosphere. Here, we consider some consequences within the distant magnetotail as the loops of disconnected flux propagate to XGSM ≈ −200 RE. The MHD simulations indicate that about an hour after intra-IMF merging events fingers of closed field lines protrude from the OCB into the polar cap. Like the observed Sun-aligned arc, these simulated auroral features grow and decay on scales of ∼10 min and have ionospheric footprints that are nearly surrounded by open magnetic flux. The simulated auroral fingers are conjugate to high-pressure channels in the distant plasma sheet. We suggest that the short-lived Sun-aligned arcs are created via an interchange process similar to that proposed by Kan and Burke [1985] to explain one class of theta auroral forms. The continuity of magnetotail currents across a high-pressure channel requires the development of field-aligned currents carried by obliquely propagating Alfven waves. Plasma drifts associated with the dusk-to-dawn electric fields of the Alfven waves are away from the Earth in the magnetotail and poleward in the nightside ionosphere. The correlation of phenomena at the nightside OCB with variations in the IMF indicate that processes other than substorms can influence boundary dynamics. Effects of self-interactions of the IMF within the dayside magnetosheath may be felt along the OCB as much as 1 hour later.

Dispersive magnetosheath‐like ion injections in the evening sector on January 11, 1997

While the plasma cloud of the January 1997 CME event was passing the Earth, the Interball Auroral Probe (Interball-2) crossed the duskside auroral oval toward a contracted polar cap. The PROMICS-3 plasma instrument observed several consecutive dispersion ramps of magnetosheath-like protons over a wide range of magnetic local times (16–21 MLT). The last dispersion ramps were observed at later local times than previously reported, likely caused by the extreme conditions during a period when the magnetosphere was immersed in the dense plasma cloud. The Polar/VIS images of the oval show a very contracted polar cap. During the first clear dispersion event energetic oxygen ions were detected. They were also observed by the the particle detectors of Polar which was in close conjugation with Interball-2 right at this moment. We suggest that the dispersion ramps are signatures of ion injections formed by impulsive entry of magnetosheath ions through the dusk-side flank into the magnetosphere during a period of strongly northward IMF and contracted polar cap.