The effect of solar wind dynamic pressure changes on low and mid‐latitude magnetic records

Abstract

Magnetic records from low and mid latitude stations have been examined to determine their response to solar wind pressure changes. The best correlation between ground level changes and the change in the square root of the solar wind dynamic pressure occurs for stations at latitudes from 15° to 30° such as Tahiti, Honolulu, San Juan and Midway. The horizontal component of the field changes on average 16.5 nT for each change of 1(nPa)½ of the square root of dynamic pressure. This is 50% greater than the vacuum model of Tsyganenko would predict for a non‐conducting Earth and therefore what would be expected for a perfectly conducting interior. Thus, low and mid latitude ground level response to solar wind pressure changes is dominated by the variation of the strength and location of the magnetopause current system and the corresponding induced currents within the Earth rather than ionospheric current sources.

Geomagnetic sudden impulses at low latitude during northward interplanetary magnetic field conditions

“…As a consequence, on the ground a sudden impulse (SI) in the geomagnetic field is observed which, at middle and low latitudes, typically involves a step-like variation of the northsouth component H whose amplitude has been found to be roughly proportional to the change of the square root of the SW dynamic pressure [Siscoe et al, 1968;Ogilvie et al, 1968;Su and Konradi, 1975]. Sis recorded by ground magnetometers at geomagnetic latitudes between 15 ø and 30 ø have been analyzed by Russell et al [1992Russell et al [ , 1994 :40), the sense of polarization of low-frequency geomagnetic field fluctuations (0.8-3.6 mHz) is CCW in the morning and reverses to CW in the afternoon but the reversal is delayed by a few hours with respect to local noon; we explained this feature, suggesting that the observed waves could be mostly generated by SW discontinuities impacting the magnetopause in the early afternoon sector, as expected for corotating interplanetary structures [see Rostoker and Sullivan, 1987], rather than by the Kelvin-Helmholtz instability, whose effects at our latitude are strongly damped [Yurnoto, 1986].…”

Some aspects of the geomagnetic response to solar wind pressure variations: a case study at low and middle latitudes

“…As previously stated, the general aspects of the asymptotic variation have been currently investigated considering the geomagnetic response of the H component approximately 10 min after the peak response (Russell et al, 1992(Russell et al, , 1994aRussell and Ginskey, 1995;. In order to allow for a comparison with previous investigations, we conducted a similar analysis considering the difference between the 10-min average, 16:05-16:15 UT, and H E .…”

“…Specific attention has been addressed in the scientific literature to the local time dependence of the geomagnetic response. Statistical analysis revealed that the asymptotic response of the H component shows at low latitudes (15 • -30 • ) a weak MLT dependence (MLT being the magnetic local time), with maximum values around local noon, minimum values around midnight, and an average value of ∼16.5 nT/(nPa) 1/2 (Russell et al, 1992(Russell et al, , 1994a. At subauroral latitudes (∼54 • -58 • ), the asymptotic response shows a different pattern, with strongly depressed (and even negative) values in the morning, and enhanced values in the afternoon (∼30 nT/(nPa) 1/2 ; Russell and Ginskey, 1995).…”

“…We focused specific attention on the correspondence between the magnetospheric field (B) observed at geostationary orbit in the noon quadrant and geomagnetic field measurements at different sites. Following previous investigations (Russell et al, 1992(Russell et al, , 1994aLe et al, 1993;Russell and Ginskey, 1995;Francia et al, 1999, we carefully examined the aspects of the MLT and latitudinal dependence of the asymptotic response.…”

Sudden Impulses in the Magnetosphere and at Ground

“…They determined, in both seasons, a maximum near midnight, a minimum at 7-8 LT, and a secondary maximum on the dayside; in addition, the amplitude of the LT modulation was much larger in the summer (approximately by a factor ≈3). Russell et al (1992Russell et al ( , 1994a reported that at low and middle latitudes  is maximum around noon during northward IMF conditions; in addition,  was found to decrease in the daytime sector and to enhance significantly in the night time sector in the case of southward IMF. At 36°, Francia et al (2001) revealed a LT dependence of the relative response, R H =/P 1/2 , characterized by a depressed value in the morning (10 nT/(nPa) 1/2 ), a greater amplitude in the evening and night sector (14-17 nT/(nPa) 1/2 ), and a maximum after the local noon (20 nT/(nPa) 1/2 ).…”