Energy time dispersion of a new class of magnetospheric ion events observed near the Earth’s bow shock
We have analyzed high time resolution (≥6 s) data during the onset and the decay phase of several energetic (≥35 keV) ion events observed near the Earth’s bow shock by the CCE/AMPTE and IMP-7/8 spacecraft, during times of intense substorm/geomagnetic activity. We found that forward energy dispersion at the onset of events (earlier increase of middle energy ions) and/or a delayed fall of the middle energy ion fluxes at the end of events are often evident in high time resolution data. The energy spectra at the onset and the decay of this kind of events show a characteristic hump at middle (50–120 keV) energies and the angular distributions display either anisotropic or broad forms. The time scale of energy dispersion in the ion events examined was found to range from several seconds to ∼1 h depending on the ion energies compared and on the rate of variation of the Interplanetary Magnetic Field (IMF) direction. Several canditate processes are discussed to explain the observations and it is suggested that a rigidity dependent transport process of magnetospheric particles within the magnetosheath is most probably responsible for the detection of this new type of near bow shock magnetospheric ion events. The new class of ion events was observed within both the magnetosheath and the upstream region.
STEREO/SEPT observations of upstream particle events: almost monoenergetic ion beams
We present observations of Almost Monoen- ergetic Ion (AMI) events in the energy range of 100- 1200 keV detected with the Solar Electron and Proton Tele- scope (SEPT) onboard both STEREO spacecraft. The en- ergy spectrum of AMI events contain 1, 2, or 3 narrow peaks with the relative width at half maximum of 0.1-0.7 and their energy maxima varies for different events from 120 to 1200 keV. These events were detected close to the bow-shock (STEREO-A&B) and to the magnetopause at STEREO-B as well as unexpectedly far upstream of the bow-shock and far away from the magnetotail at distances up to 1100RE (STEREO-B) and 1900RE (STEREO-A). We discuss the origin of AMI events, the connection to the Earth's bow- shock and to the magnetosphere, and the conditions of the in- terplanetary medium and magnetosphere under which these AMI bursts occur. Evidence that the detected spectral peaks were caused by quasi-monoenergetic beams of protons, he- lium, and heavier ions are given. Furthermore, we present the spatial distribution of all AMI events from December 2006 until August 2007.
A review of upstream and bow shock energetic-particle measurements
SummaryMeasurements of energetic electrons and protons frequently observed in the interplanetary medium upstream from the bow shock are reviewed. Bursts of electrons with energy 0.5 to 100 keV and duration 1 to 100s (termed “spikes”) appear when the spacecraft is on a field line which is tangent to the bow shock surface. The electrons move upstream in a thin, curved sheet lying nearly along the interplanetary-magnetic-field direction. The thinness of the electron sheet shows that the electrons are accelerated only in a small region about the curve of tangency of the interplanetary magnetic field and the bow shock surface. The upstream ions, on the other hand, occupy a much larger spatial region. Some of the upstream ions, particularly those at the energies above 100 keV, may have magnetospheric origin. The mechanism of the ion acceleration is still not known nor is it ruled out that acceleration in the upstream region may take place. A variety of plasma waves is excited by the upstream particles.RiassuntoSi riesaminato misurazioni di elettroni e protoni energetici frequentemente osservati nel mezzo interplanetario a monte dell'onda d'urto. Esplosioni di elettroni con energia da 0.5 a 100 keV e durata da 1 a 100 s (chiamate «spikes») appaiono quando la navicella spaziale è su una linea di campo che è tangente alla superficie dell'onda d'urto. Gli elettroni si spostano a monte in uno strato sottile e curvo che giace quasi lungo la direzione del campo magnetico interplanetario. La sottigliezza dello strato elettronico mostra che gli elettroni sono accelerati solo in una piccola regione intorno alla curva di tangenza fra il campo magnetico interplanetario e la superficie dell'onda d'urto. Gli ioni a monte, d'altra parte, occupano una regione di spazio molto più ampia. Alcuni di questi ioni a monte, particolarmente quelli a energia superiori a 100 keV, possono avere origine magnetosferica. Il meccanismo di accelerazione degli ioni non è ancora noto, nè è escluso che nella regione a monte possano verificarsi accelerazioni. Una certa varietà di onde di plasma è eccitata dalle particelle a monte.РезюмеАнализируются свойства энергетичных энектронов и протонов, наблюденных в межпланетной среде и (по крайней мере в течение многих часов) ускоренных либо в, либо вдоль поверхности ударных волн. Наблюдались усиления потоков электронов с энергией от 0.5 до 100 кэВ с малой длительностяю (от секунд до минуты), когда космический аппарат находится на линии магнитного поля, которая касается ударной волны сжатия. Эти всплески обычно имеют место, когда интенсивность протонов сходных энергий либо увеличивается, либо уменьшается; а также во времена, когда протонный поток остается, по существу, постоянным в течение многих минут или даже часов. Эти волны возбуждаются в межпланетной среде частицами больших энергий.