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A New Way to Study Geomagnetic Storms RISHBETH PRIZE • HUTCHINSON ET AL.: GEOMAGNETIC STORMS RISHBETH PRIZE • HUTCHINSON ET AL.: GEOMAGNETIC STORMS A new way to study geomagnetic storms James Hutchinson and colleagues describe a novel radar technique to study geomagnetic storms: superposed latitude–time–velocity plots. This is a summary of the poster winning a Rishbeth Prize at the NAM/UKSP/MIST meeting in April. Downloaded from https://academic.oup.com/astrogeo/article/52/4/4.20/218024 by guest on 30 September 2021 pace is not empty. The solar wind (SW) plasma on these field lines. Auroral images are 12 carries millions of tonnes of energetic par- dayside plasmasphere also often used as a proxy for the size of the Sticles outward from the Sun each second cusp open region of the magnetosphere via the dim which, along with the embedded interplanetary boundary region (or polar cap) within the main aurora and magnetic field (IMF), exerts a strong influence layer can be used to find the open–closed boundary on our own geospace. Usually we are protected (OCB). A sudden broad brightening (or expan- by our own magnetic shield called the magneto- 18 06 sion) in the pre-midnight sector and subsequent sphere. However, geomagnetic storms – defined recovery of the auroral oval, is also a clear indi- by periods of intense solar wind–magnetosphere cation of the substorm cycle: the fundamental coupling in association with extreme SW condi- global disturbance of the Dungey cycle where tions, usually coronal mass ejections (CMEs) or polar cap plasma sheet stored energy in the magnetotail from prolonged ring current co-rotating interaction regions (CIRs), which 24 magnetic reconnection on the dayside is sud- cause large global disturbances in the Earth’s 1: View looking down on the northern high- denly and explosively released every few hours magnetosphere (Gonzalez et al. 1994) – deposit latitude ionosphere, labelling the footprints causing vivid auroral displays. However, times huge amounts of energy into the magnetosphere. of key aspects of the magnetosphere and of extreme solar wind–magnetosphere coupling One manifestation of this is the aurora (or north- showing the plasma flows in solid arrows in can cause reconnection at a rate far greater than ern lights), a ring or oval of emission centred the classic twin cell convection pattern of the can be recovered by the substorm cycle, forming Dungey cycle. (Adapted from Cowley 1993) on the magnetic poles when viewed from space, a geomagnetic storm that among other effects caused by the interaction of the neutral atmos- causes enhanced auroral displays, radiation belts phere and energetic particles trapped in the near-Earth region. However, the two do inter- and ring current, with the latter particularly use- Earth’s magnetic field. act: the solar wind crashes into the dayside ter- ful in the identification of these storms. We use a Recent headlines such as “Space storms restrial field causing it to be compressed and combination of datasets to present a new super- threaten technology” (http://www.bbc.co.uk/ then flows around the Earth’s field, dragging it posed epoch analysis technique for SuperDARN news/world-us-canada-12516918, accessed into an elongated tail-like structure called the radar data during geomagnetic storms. 15/06/2011) and “Solar storms could cre- magnetotail. Under certain conditions, usually ate $2 trillion ‘global Katrina’ ” (http://www. associated with a southward orientation of the Identifying geomagnetic storms guardian.co.uk/science/2011/feb/21/solar- IMF (usually quoted in three components: X in Geomagnetic storms are identified via a char- storms-global-katrina, accessed 15/06/2011) the Sun–Earth line, Z is in the direction of the acteristic SYM-H index trace (figure 2), a high- highlight the importance of understanding this northern magnetic pole and Y perpendicular to cadence index to observe geomagnetic activity interaction. With society’s ever growing depend- both), a process called magnetic reconnection created using a network of magnetometers ence on satellite applications such as the Global can occur, opening the Earth’s dipole field to positioned around the Earth’s equator to meas- Positioning System, satellite communications the IMF and allowing the transport of energetic ure tiny deflections in the terrestrial magnetic and TV, along with the threat of damage to solar particles into the system. These open field field strength caused by the induced field from our national power grids through geomagnetic lines and plasma convect across the magnetic the ring current – the flow of ions and electrons induced currents (GICs, Turnbull 2010), the poles with the flow of the solar wind and are around the “equator” of near-Earth space. The need to understand and predict the effects of stored in the magnetotail until a second point SYM-H index trace consists of three distinct space weather are of vital importance. of magnetic reconnection occurs, reclosing the phases: initial, main and recovery phase. The terrestrial field and the field lines convect back initial phase is a small positive increase in The magnetosphere, magnetic to the dayside at low latitudes. SYM-H associated with the squeezing of the reconnection and the Dungey cycle This repeatable cycle is known as the Dungey dayside terrestrial magnetic field by increased The magnetosphere is a region in spaced formed cycle (1961) and repeats with the natural varia- SW ram pressure from the solar eruption or by the draping of the SW and IMF on our own tion in the IMF from positive to negative values fast stream. With the onset of favourable IMF terrestrial dipole magnetic field and ionospheric of BZ, resulting in the footprints of the magnetic conditions, namely a southward IMF orienta- plasma (see figure 1 in “Magnetic fields in a field lines tracing a well known twin-cell convec- tion, enhanced dayside magnetic reconnection flap!” on page 4.17 of this issue). Initially the tion pattern (figure 1) that can easily be observed can occur, enlarging the polar cap, and in turn magnetosphere was considered a closed system, by the Super Dual Auroral Radar Network convecting large amounts of energy into the meaning solar wind plasma could not enter the (SuperDARN) via backscatter off the flows of magnetotail, exciting the radiation belt and ring- 4.20 A&G • August 2011 • Vol. 52 RISHBETH PRIZE • HUTCHINSON ET AL.: GEOMAGNETIC STORMS RISHBETH PRIZE • HUTCHINSON ET AL.: GEOMAGNETIC STORMS current plasma. This increase in ring-current 2: The characteristic particle energy and density induces a magnetic initial main recovery SYM-H index trace of 100 phase phase phase field that opposes the terrestrial one, causing a a geomagnetic storm sharp negative drop in the SYM-H index and showing the initial, main identifies the main phase. The recovery phase and recovery phases 0 is shown in the gradual return of SYM-H to normal values via ring-particle losses (usually –100 taking days) and scattering out of the system; the onset of recovery is usually associated with SYM-H index (nT) a reduction in the enhanced SW driving condi- –200 tions or a positive or northward tuning of the IMF B component. Z –300 16.00 00.00 08.00 16.00 00.00 08.00 Unanswered questions… UT There are still several unanswered questions Downloaded from https://academic.oup.com/astrogeo/article/52/4/4.20/218024 by guest on 30 September 2021 concerning geomagnetic storms, particularly in the relationship between duration and size 3: Diagram of the superposition method stretched of storms, important in understanding space using a normalized timeline. timestamp averaged weather effects as well as the mechanisms of normalized timeline excitation and decay of the ring current. Here we are addressing the first question, using a varied storm durations compressed with data in 2-minute timestamp combination of radar data and auroral images blocks as a proxy for the amount of coupling and energy input to the magnetosphere during dif- ferent intensity storms. This is in addition to a 4: Example of +Z (5 nT) superposed 2-minute previously completed statistical study of 143 +Y geomagnetic storms over the most recent solar period of radar 3 cycle (1997–2008) and their associated SW con- backscatter using (–30 min) the map potential ditions through parameters such as SW speed, technique on 3 pressure, density and IMF components which geomagnetic storm APL model showed that both the relative size and duration periods discussed. -9 6 < BT < 12 of SW enhancement was important in deter- Bz- -33 mining how large a storm can become. This -21 1000 15 means that the most intense storms can occur 900 3 800 on the same timescales as the weakest, making -9 -21 velocity (ms 700 the prediction of hazardous events difficult. We 600 superpose events to the average duration of each 500 phase (initial, main and recovery) for each storm 400 –1 ) size category rather than a simple t0 start time 3 300 alignment, essentially stretching and compress- 200 –1 100 ing the timestamp of the data to give a better 1000 ms 60 kV alignment of “like” features in the naturally northern hemisphere variable storm progression (seen in figure 3). SuperDARN and IMAGE 5: Example of 40 SuperDARN (Greenwald et al. 1995) is an superposed auroral international array of high-frequency coherent- images to match the 2-minute period of scatter radars, currently consisting of 19 north- 900 figure 4 from the 20 ern and 9 southern hemisphere radars with their IMAGE spacecraft. 800 fields-of-views (FOV) covering a significant pro- cameraSI-12 counts portion of both auroral and polar regions of the 700 ionosphere (each FOV has an estimated area 600 2 of ~4 million km ). Doppler-shifted backscat- 0 500 ter is received from ionospheric plasma irregu- 400 larities when the Bragg scattering condition is met, with autocorrelation functions constructed co-latitude (degrees) 300 to extract parameters such as signal-to-noise –20 200 power, velocity and spectral width.
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