Geostationary Field Research Articles

Comparative statistical analysis of storm time activations and sawtooth events

Statistical properties of storm time magnetospheric activity are examined using superposed epoch analysis. We show that about half of storm time auroral electrojet activations have signatures that are typical of nonstorm substorms, including geostationary orbit injections and magnetic field dipolarizations. Analysis of a separate data set of sawtooth events shows that they have auroral and inner magnetosphere characteristics that are quite similar to those found generally during storm time activity. Hence it is concluded that the sawtooth events do not represent a specific class of magnetospheric activity. Examination of the solar wind and IMF properties showed that about 30% of storm time substorm‐like activations and about 20% of the sawtooth oscillations have associated solar wind or IMF triggers and that triggering is more likely during high solar wind pressure and fluctuating IMF. The solar wind‐magnetosphere coupling efficiency is shown to be independent of the solar wind Mach number or level of IMF fluctuations but dependent on the level of driving; when EY is small, the ionospheric dissipation, ring current intensification, and geostationary field stretching are relatively larger than when the driving EY is large.

A statistical investigation of dayside magnetosphere erosion showing saturation of response

We present a statistical investigation of dayside magnetospheric erosion based on a survey of 316 cases in 1996–2004. We monitor erosion through the depression in the strength of the terrestrial magnetic field at geostationary heights using magnetic field observations acquired by four GOES spacecraft 2 hours on either side of local magnetic noon. The southward component of the interplanetary magnetic field (IMF) and the solar wind dynamic pressure, quantities which are responsible for opposing effects on the geostationary field, are obtained from the Wind and ACE spacecraft. We extend our previous work to encompass the Bz range [0, −35] nT. We find that dayside erosion saturates when IMF Bz is in the range [−12, −16] nT. This result is consistent with global simulations. With the measured solar wind velocity, this corresponds to an interplanetary electric field (IEF) of 6.2 ± 1.6 mV m−1. We discover that the maximum depression of the geostationary field at saturated erosion is ∼26 nT. In addition, we reveal a direct relation between the saturation of flux erosion of the dayside magnetosphere and that of the cross‐polar cap potential (CPCP), both of which occur at approximately the same electric field. For the latter, we apply the Hill‐Siscoe model for the calculation of CPCP to our data set. We find the CPCP to deviate from a linear increase with IEF at ∼4 and 7 mV/m, which correspond closely to the IEF range at which we find saturation of dayside erosion to start to manifest itself.

The geostationary field during dayside erosion events 1996–2001: A joint Wind, ACE, and GOES study

We carry out a statistical investigation on dayside magnetosphere erosion during the period 1996–2001, using Wind, ACE, and GOES observations supplemented by ground‐based magnetograms. The inner magnetospheric signature of erosion that we focus on is the depression of the geostationary magnetic field strength (ΔBtot) within 2 hours of local magnetic noon. We relate this decrease to the southward component of the interplanetary magnetic field (IMF) in the range −16 ≤ Bz ≲ 0 nT and the solar wind dynamic pressure in the range ∼0–6 nPa. Thus both relevant interplanetary parameters extend to well beyond average values at 1 AU. The geostationary field strength (Btot) is derived from measurements by the GOES 8, 9, and 10 spacecraft. Our data set consists of 288 measurements. We adopt two approaches. In one approach, we first correct for the effect of dynamic pressure on the geostationary field strength by studying days with various dynamic pressure values but low IMF clock angles, θ (<45°). With the latter condition we mean to exclude dayside reconnection to a reasonable degree. We then use this pressure correction to infer the erosion contribution to Btot (ΔBtot). We find ΔBtot[nT] = −3.58 + 1.67Bz with a correlation coefficient of 0.59. This analysis is most reliable in giving the change of the geostationary field to a decrease in IMF Bz, for which we obtain ΔBtot/Bz = −1.67 nT per 1 nT decrease in Bz. In the second approach, we do a bivariate fit of Btot to the (Bz, Pdyn) data pairs and obtain Btot[nT] = 97.64 + 20.22 + 1.56Bz. Major sources of error are discussed, and the results are compared to previous work.

Dayside erosion during intervals of tenuous solar wind

We present six data examples where we infer erosion of the dayside magnetosphere during intervals of very tenuous solar wind (density < 1 cm−3). The interplanetary observations were made by the Wind spacecraft when the average solar wind dynamic pressure Pdyn and the interplanetary magnetic field (IMF) Bz were in the ranges (0.07, 0.62) nPa and (−7.6, −0.9) nT, respectively. The inner magnetospheric signature of erosion we focus on is a decrease in the strength of the geostationary magnetic field, as monitored by NOAA's GOES spacecraft. We obtain this decrease as a function of IMF Bz by comparing each event with a reference day, May 11, 1999. During the reference day the lowest Pdyn of the set was attained (0.07 nPa), IMF Bz > 0, and the geomagnetic field at geostationary orbit was dipolar. The central point we make is that although compared to the reference day the Pdyn in each event is higher, the strength of the geostationary field is weaker. We interpret this as evidence that the field compression due to Pdyn has been overcome by the field depression due to erosion. Correcting empirically for the compression of the geostationary field due to solar wind dynamic pressure, we find that for the tenuous solar winds we consider the decrease of the geostationary field, ΔBGS, is related to IMF Bz as ΔBGS (nT)= −2.8 + 2.3 Bz (nT).