Direct observations of injection events of subauroral plasma into the polar cap

Abstract

While polar cap ionospheric patches have been studied for over two decades, there remains no general agreement to which of many proposed patch‐production mechanisms are important or dominate. An experiment was designed and implemented to search for transient events redirecting subauroral ionospheric plasma from its subauroral flow to transient injection into the polar cap, as would occur for the (Lockwood and Carlson, 1992) mechanism of patch creation. An earlier experiment provided compelling evidence of this mechanism acting within the cusp, with strong but indirect evidence regarding the source‐reservoir for the plasma injected into the polar cap. The work here, for the first time, directly tracks plasma becoming a “patch”, continuously from subauroral latitudes before the event fires, through the cusp and into the polar cap. We conclude this mechanism is a dominant patch generation mechanism and highlight that poleward‐moving‐form research has direct application to polar cap patch generation by magnetopause reconnection.

On a new process for cusp irregularity production

“…These data included simultaneous observations of north-south meridian scans with the ESR, the Tromsø EISCAT VHF and UHF radars looking north at low elevation near the ESR field-of-view, and optical data from all-sky imaging photometers (ASIPs) and meridian scanning photometers (MSPs). Carlson et al (2006) reported a sequence of five successive patches and mapped them into the polar cap during a 40 min interval near local magnetic noon, reproduced here in Fig. 2.…”

“…These data show five successive patches that are created and injected across the cusp and into the polar cap. Further details are given in Carlson et al (2006), which is the source of this figure.…”

“…In summary, after developing in 2001 an observing mode for the ESR that was capable for the first time of testing the Lockwood and Carlson (1992) mechanism, Carlson et al (2004Carlson et al ( , 2006 established that this magnetic reconnection based mechanism, is a dominant mechanism for creating polar cap patches. The same data body showed associated strong flow shears to be commonly observed in the cusp.…”

EISCAT observations of plasma patches at sub-auroral cusp latitudes

“…Several observation techniques have been used for patch studies, such as direct electron density measurements by ionosonde (Buchau et al, 1983;Dandekar and Bullett, 1999;Dandekar, 2002), incoherent scatter radar (e.g. Valladares et al, 1994Valladares et al, , 1996Valladares et al, , 1998Carlson et al, 2002Carlson et al, , 2004Carlson et al, , 2006Lockwood and Carlson, 1992;Lockwood et al, 2005aLockwood et al, , 2005bOksavik et al, 2006a), and ionospheric tomography (Walker et al, 1999;Pryse et al, 2004), 630.0-nm airglow intensity that is proportional to electron density (Weber et al, 1984;Harris, 1995, 1996;McEwen et al, , 2004Lorentzen et al, 2004), and more indirectly by HF coherent scatter radars of the SuperDARN network (Rodger et al, 1994;Ogawa et al, 1998;Rodger and Graham, 1996;Milan et al, 2002Milan et al, , 2005Oksavik et al, 2006a). The latter technique does not provide a measurement of the electron density but gives the backscatter power from irregularities on electron density gradient associated E×B drift instability (Tsunoda, 1988;Chaturvedi and Ossakow, 1981).…”

“…-Plasma structuring by transient reconnection where the open closed boundary (OCB) leaps equatorward to a region of higher density plasma, followed by poleward relaxation of that boundary carrying with it the high density plasma accelerated into the polar flow (Lockwood and Carlson, 1992;Carlson et al, 2002Carlson et al, , 2004Carlson et al, , 2006.…”