Magnetic signatures of medium‐scale traveling ionospheric disturbances as observed by CHAMP

Abstract

In this work we analyze the global distribution and physical characteristics of nighttime midlatitude magnetic field fluctuations (MMFs) as observed by the CHAMP satellite from 2001 to 2002 (solar maximum) and from 2006 to 2007 (solar minimum). MMFs are defined as medium‐scale magnetic fluctuations perpendicular to the mean field, which are not accompanied by plasma density irregularities at the CHAMP altitude (∼400 km). MMFs occur at 15°–40° invariant latitude in the ionospheric F region. The occurrence is rare above the southern Atlantic ocean, and bears little connection to geomagnetic activity. The global MMF occurrence rate depends on season. The occurrence is generally low in equinox, maximizes around east Asia/Oceania and Europe/northern Atlantic Ocean in June solstice, and peaks above the American continents in December solstice. As the solar cycle declines, the detected MMF occurrence rate also decreases. The MMF occurrence peaks around 2100 LT and slowly decreases toward midnight. In the postmidnight sector, events are practically absent. The MMF occurrence is generally consistent with known features of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs), such as the conjugate climatology, and premidnight occurrence peak in the east Asia/Oceania region. But differences in their distributions also exist, implying that factors other than MSTIDs, e.g., ionospheric conductivity, sporadic E layer or plasma instabilities, may play a nonnegligible role in generating MMFs. MMFs have a preferred direction of polarization, which is consistent with that of MSTIDs and again corroborates the close connection between these two phenomena. We interpret the observed magnetic deflections in terms of field‐aligned currents (FACs). The estimated wavelength range (∼200–500 km) of associated FAC pairs also agrees well with the size of MSTID density structures.

The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite

“…Pottelette et al (2007) estimated the FAC density for one EPB example and reported the value 0.2 µA/m 2 , which is well in line with our results. It is also interesting to note that the obtained amplitudes are comparable to FAC densities associated with MSTIDs (Saito et al, 1995;Park et al, 2009).…”

“…The resulting "residual" field is then transformed into the mean-field-aligned (MFA) coordinate system, where the z-component, "parallel," is along the mean field, the y-component, "zonal," is perpendicular to the magnetic meridian and is pointing to the east, and the x-component, "meridional," completes the triad and points outward. A similar approach was used in Stolle et al (2006), Park et al (2008), and Park et al (2009). Remaining largescale variations of order 10 nT, which are not properly allowed for by POMME4, are further eliminated with a 91-point (∼700 km) median filter in Sects.…”

“…These events are located outside the Appleton anomaly. Therefore they might not be EPBs but mid-latitude magnetic fluctuations (MMFs) connected with medium-scale traveling ionospheric disturbances (MSTIDs) as reported by Park et al (2009). In this work we will focus only on EPBs which are identified with fluctuations in the parallel component of the magnetic field.…”

Global characteristics of Pc1 magnetic pulsations during solar cycle 23 deduced from CHAMP data

“…Stolle et al, 2006) or from medium-scale traveling ionospheric disturbance (MSTID) (e.g. Park et al, 2009). Conversely, an inherent drawback of this method is the systematic exclusion of Pc1 events within the auroral oval due to their collocation with small-scale FAC structures (e.g.…”

“…For the transformation into the MFA frame we do not use the traditional and rigorous approach, which starts from 1 Hz geomagnetic field data in north-east-center (NEC) coordinates and uses geomagnetic field models to define the "mean field" (see, for example, Park et al, 2009, for more details). We actually employ magnetic field readings in their sensor frame.…”