JapaneseGEMSIS-Ionosphere
Sudden Impulse
Geomagnetic Field Variations Produced by 3-D Current System during Sudden Compressions
The sudden compression (SC) of the magnetosphere associated with an abrupt increase in solar wind dynamic pressure leads to enhancements in magnetospheric convection in the inner magnetosphere and the development of field-aligned currents (FACs) resembling Region-1 FACs and the resultant ionospheric currents. In order to clarify magnetic latitude and local time profiles of geomagnetic field variations from subauroral latitudes to the equator, we performed a statistical analysis of SC amplitude using long-term observational data obtained from eight stations. As a result, the SC amplitude profiles in the subauroral and middle latitudes showed magnetic field variations produced by DP2-type currents in the daytime sector. The effects of the DP2-type currents expanded at least to the low latitude (magnetic latitude: 16.54°). However, the SC amplitude in this region indicated that the DL part of SC produced by magnetopause current was much more dominant. On the daytime equator, the SC amplitude was considerably enhanced by an intensification of the Pedersen current, driven by the penetration of the convection electric field due to the Cowling effect. Also of interest is that SC amplitude in the nighttime sector tended to be enhanced from the mid-latitude to the equator, and its peak value increased with increasing magnetic latitude. This result suggests that the magnetic field variations produced by the FACs during the main impulse phase of SCs expanded to the equator.
Magnetic Latitude and Local Time Dependence of Sudden Commencement Amplitude
In order to investigate magnetic latitude and local time dependence of sudden commencement (SC) amplitude in a region from the magnetic equator to mid-latitude, we analyzed geomagnetic field variations of the H component obtained from four stations (Yap, Guam, Okinawa, Kakioka, and Memanbetsu). Here we defined SC events as magnetic field disturbances indicating an abrupt increase of amplitude and time variation of more than 5 nT and 1.5 nT/min, respectively, within a period from January 1981 to January 2008. The diurnal variation of SC amplitude during a main impulse (MI) phase showed a DP2-type variation with its minimum and maximum value around the morning and evening sectors, respectively, in the daytime sector of the middle latitude. At low latitudes, the SC amplitude showed a magnetic field variation originating from the magnetopause current, with its maximum value around noon. Moreover, the diurnal variation near the magnetic equatorial region showed strong enhancement around 10 h, due to the Cowling effect associated with the penetration of the polar electric field to the equator. The SC amplitude in the nighttime sector showed a secondary peak around midnight at all stations, indicating that the amplitude increases with increasing magnetic latitude. It seems that the magnetic field disturbances are produced by the region-1 type field-aligned currents during the MI phase.
Reference: Shinbori et al., JGR, 2012
Anomalous Occurrence of the Preliminary Impulse of Geomagnetic Sudden Commencement in the South Atlantic Anomaly Region
We found that the preliminary reverse impulse (PRI), which is rarely seen in low latitudes, frequently appears (occurrence probability of 80%) near the center of the South Atlantic Anomaly (SAA) region. From calculations of ionospheric conductivity derived from the IRI-2007 and NRLMSISE-00 models, we showed that the height-integrated conductivity was more enhanced in the SAA region, where the ambient magnetic field intensity was weak. The significant enhancement of the PRI occurrence is caused by the increased ionospheric conductivity in this region.
Reference: Shinbori et al., JGR, 2010
Upward Poynting flux from the Ionosphere to the Inner Magnetosphere
Based on spacecraft observations, the electric field associated with the preliminary impulse of the geomagnetic sudden commencement was found to be transmitted from the ionosphere to the inner magnetosphere, with an upward flow of the Poynting flux. The upward Poynting flux matches the Earth-ionosphere waveguide model in which the polar electric field propagates to low latitudes at the speed of light, and the ionospheric electric field further propagates upward into the inner magnetosphere.
Reference: Nishimura et al., JGR, 2010
Proton Aurora Associated with Magnetic Impulse Events
Auroral hydrogen emission at 486.1 nm (proton aurora) associated with a magnetic impulse event (MIE) was observed for the first time by the all-sky imager installed at the South Pole Station (-74.3° MLAT). Optical observations at the South Pole Station showed a clear spot-like brightening of proton auroral emissions associated with MIEs. These proton aurora spots were ~300-500 km in length and ~150-200 km in width at an altitude of 150 km and lasted for ~1-2 minutes. The spot drifted antisunward with a speed of ~3-5 km/s, but did not always drift. The proton precipitation may be a result of the adiabatic change in the proton’s pitch angle in the magnetosphere or the direct penetration of solar wind protons.
Reference: Ebihara et al., JGR, 2010
Variation of Ionospheric Plasma Convection Associated with a Negative SI
Spatial-temporal evolution of ionospheric plasma convection, associated with a negative sudden impulse (SI) signature on ground magnetograms, has been examined in detail using SuperDARN HF radar, magnetometers, and satellite observations over the ground measurements. We recently discovered that transient eastward flows in the ionosphere shifted poleward in the subauroral region with finite latitudinal widths of several degrees. These results were quantitatively consistent with the poleward propagations of geomagnetic variations, observed directly under ionospheric flows. The fact that the poleward-shifting eastward flow was observed repeatedly with decreasing speed implied that a sudden expansion of the magnetosphere, inducing an SI signature on the ground, produced global damping oscillation of the magnetosphere with a rapidly decreasing amplitude. As a result, the electric field and current generated in the ionosphere due to the magnetospheric oscillation was interpreted by the radar and ground magnetometers as a magnetosphere-ionosphere coupling process.
Reference: Hori et al., JGR, 2012