H. Kawano

Annual and semiannual variations of the location and intensity of large-scale field-aligned currents

[1]xa0The present study examines seasonal variations of large-scale field-aligned current (FAC) systems in terms of the dipole tilt and clock angles. Magnetic field measurements from the DMSP F7 and F12-F15 satellites are used. This data set consists of a total of ∼185,000 auroral oval crossings, out of which ∼121,000 crossings were selected for the present analysis. Focus is placed on the latitude at the demarcation between the region 2 (R2) and region 1 (R1) currents and the intensities of these currents. It is found that the dayside FAC moves poleward and equatorward in the summer and winter hemispheres, respectively, and the nightside FAC has the opposite seasonal dependence. In the midday sector the peak-to-peak variation of the FAC latitude over the entire range of the dipole tilt is ∼5°, whereas it is ∼4° around midnight. In the flank sectors the average FAC latitude is higher around the solstices than around the equinoxes irrespective of hemisphere. The corresponding dependence on the dipole clock angle can actually be found for almost all local time sectors, although the peak-to-peak variation of the expected semiannual variation, 2° around noon and <1° in other local time sectors, is smaller than that of the annual variation except for the flank sectors. A comparison with a model magnetic field strongly suggests that the dipole tilt effect on the magnetospheric configuration is the primary cause of the annual variation, whereas the semiannual variation is inferred to reflect the fact that geomagnetic activity tends to be higher around the equinoxes. The average dayside FAC intensity is larger in the summer hemisphere than in the winter hemisphere, which can be explained in terms of the seasonal variation of the ionospheric conductivity. The dayside R1 current intensity depends more strongly on the dipole tilt than the dayside R2 current intensity, and it changes by a factor of 2–3 over the entire range of the dipole tilt angle. In contrast, the annual variation of the nightside FAC intensity is more complicated, and the nightside R2 current seems to be more intense in the winter hemisphere than in the summer hemisphere. The dependence of the FAC intensity on the dipole clock angle is less significant especially for the R1 system. Nevertheless, the result suggests that the FAC tends to be more intense around the equinoxes, which is consistent with the semiannual variation of geomagnetic activity.

Dependence of flux transfer events on solar wind conditions from 3 years of Cluster observations

We investigate the dependence of Cluster high-latitude magnetopause and low-latitude flank flux transfer events (FTEs) on solar wind conditions using measurements from Cluster FGM and CIS and ACE MFI and SWEPAM between February 2001 and June 2003. We find that there are strong dependences of Cluster FTE occurrence on the IMF B-xgsm, B-ygsm, and B-zgsm components but not on the IMF magnitude. There are strong dependences of Cluster FTE occurrence on the IMF clock, tilt, spiral, and cone angles. However, some patterns are significantly different from previous results. The solar wind density, speed, Beta, VxBz, dynamic pressure, and magnetosonic Mach number have different degrees of control on FTE occurrence. FTE separation time is found to be clearly controlled by IMF B-ygsm, B-zgsm, and magnitude, and the IMF clock, tilt, spiral, and cone angles, and weakly controlled by the solar wind VxBz and magnetosonic Mach number. There is no obvious control of it by other IMF and solar wind parameters. FTE peak-peak magnitude is found to be controlled by IMF B-ygsm, B-zgsm, and magnitude and by the solar wind density and dynamic pressure but not by other IMF and solar wind parameters. The FTE dawn-dusk asymmetry is not likely caused by the Parker spiral IMF. Some FTE statistical patterns are strongly dependent on FTE locations. Finally, we see similar to 4% of the FTEs corresponding to a single change in IMF B-zgsm from positive to negative, similar to 4% corresponding to a single change from negative to positive, and similar to 43% corresponding to multiple changes in the sign of IMF B-zgsm, all within the 10-min interval preceding the FTEs. There is still no evidence for a direct correlation between IMF B-zgsm changing sign and FTEs.

Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes

[1]xa0In order to investigate the characteristics of Pi 2 propagation observed in the high-latitude region, ground magnetometer data obtained at high-latitude CPMN stations were analyzed. The power of magnetic perturbation, (ΔH)2 + (ΔD)2, were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotelnyy (KTN: MLAT = 69.94°, MLON = 201.02°) reached the maximum amplitude earlier than those at lower-latitude station Tixie (TIK: MLAT = 65.67°, MLON = 196.88°), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67°, MLON = 212.12°) and Kotzebue (KOT: MLAT = 64.52°, MLON = 249.72°). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been ∼22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward (westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT (set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210° magnetic meridian. Our results show that the low-latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high-latitude Pi 2s in addition to low-latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher-latitude region, for studying substorm onset timing issues.

Dual-satellite observations of the motions of flux transfer events: Statistical analysis with ISEE 1 and ISEE 2

[1]xa0Flux transfer events (FTEs) are transient phenomena observed near the magnetopause, characterized by bipolar perturbations in the magnetic field component perpendicular to the magnetopause (BN). FTEs are generally thought to be the results of transient reconnection at the magnetopause. In this paper we address the direction of motion of FTEs, which is difficult with one satellite, by statistically analyzing FTEs simultaneously observed by ISEE 1 and ISEE 2 satellites, which made observations for continuous 10 years (1978–1987). Two-point observation does not completely resolve the direction of motion; we cover it by large-scale statistics (634 events) of the 10-year data of ISEE. For each FTE we have determined the time lag from ISEE 1 to ISEE 2 as that which maximizes the cross-correlation function between BNs at ISEE 1 and ISEE 2. This time lag has a significant correlation with the longitudinal distance between the two satellites, consistent with east-west motions away from the subsolar point. The time lag shows a weaker correlation with the latitudinal distance. The significant east-west motion suggests that FTEs in nature are likely to have a structure whose longitudinal scale is shorter than its latitudinal scale. In other words, flux rope structure is more consistent with the observations than a long-X-line type structure.

AKR modulation and global Pi2 oscillation

[1]xa0In this report we present a temporal relationship between ground Pi2 and auroral kilometric radiation (AKR). We analyzed six isolated substorm events, which were observed by the MAGDAS/CPMN ground magnetometer network and the plasma wave instrument onboard the Polar satellite. We found that the time derivative of the height-integrated AKR power and the ground Pi2 D component had the same periodicity and that the two were synchronized with each other. When the D component fluctuated with the same (opposite) polarity as the magnetic bay variation, the AKR power tended to increase (decrease) during the corresponding interval. An isolated substorm event (AE ∼ 40 nT), which occurred around 10:19 UT on 24 January1997, was selected for a detailed study. The behavior of the Pi2 event can be interpreted by the substorm current wedge (SCW) and Pi2 propagation models. It is confirmed that the midlatitude and high-latitude D component oscillations can be treated as a proxy of the SCW oscillations, whereas the H component oscillations exhibited some phase shifts by the propagation delay of the Pi2 waves. That is, the temporal relation between the time derivative of the AKR power and the ground Pi2 suggests that the height-integrated AKR power was modulated coherently with the SCW oscillations.

Anomalous geomagnetic variations associated with the volcanic activity of the Mayon volcano, Philippines during 2009–2010

Abstract Local anomalous geomagnetic variations preceding and accompanying the volcanic eruptions had been reported by several researchers. This paper uses continuous high-resolution geomagnetic data to examine the occurrence of any anomalous geomagnetic field variations that possibly linked with the volcanic eruption of the Mayon volcano, Philippines during 2009–2010. The nearest geomagnetic observing point from the Mayon volcano is the Legazpi (LGZ) station, Philippines; which is located about 13 km South of the Mayon volcano. The amplitude range of daily variations and the amplitude of Ultra Low Frequency emissions in the Pc3 range (Pc3; 10–45 s) were examined at the LGZ station and also were compared with those from the Davao (DAV) station, Philippines as a remote reference station. Both the LGZ and DAV stations belong to the MAGDAS Network. The result of data analysis reveals significant anomalous changes in the amplitude range of daily variations and the Pc3 amplitude at the LGZ station before and during the volcanic eruption of the Mayon volcano. From the obtained results, it appears that the observed anomalous variations are dependent on the change in the underground conductivity connected with variation in the physical properties of the Earth’s crust due to the activity of the Mayon volcano. Therefore, these anomalous geomagnetic variations are considered to be of a local volcanic origin.