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The Bastille Day Shock

The Bastille Day Shock

On July 14, 2000, an enormous x-class flare was observed near the center of the solar disk of the Sun (a-b).

a. Image
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b. Image
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Left: This SOHO animation of the July 14th X-class solar flare was recorded by the spacecraft's Extreme-ultraviolet Imaging Telescope at 195 angstroms. We can see the flare, followed by a torrent of energetic particles that arrived about 15 minutes later, creating snow on the images as the particles bombarded the camera's electronic detectors. Courtesy of SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.
Right: Solar Flares are classified by their x-ray flux in the 1.0 - 8.0 Angstrom band as measured by the NOAA GOES-8 satellite. On July 14, 2000, a solar flare from active region 9077 registered as a powerful X5-class eruption. Another X-class flare from 9077 was recorded on July 12, 2000.

An x-class flare is the most intense flare recorded and, like smaller flares, is thought to be the result of reconnection at the base of the solar corona.

The Bastille Day flare may have been produced by a larger, more violent and active version of the reconnection event being shown in this movie (c).

c. Image
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This TRACE sequence of ultraviolet images shows a flare at the solar limb in a chromospheric line of hydrogen. Hot plasma fills the loops and then falls back down the loops to the limb. A flare is an energetic release of electromagnetic radiation (light) and particles.

Shortly after the flare was observed, an enormous full halo CME was observed by several solar imaging instruments, some located on spacecraft. The CME drove an extremely fast shock ahead of it.

This simulation shows a CME driving a 2D shock into the interplanetary medium (d).

d. Image
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The enormously extended, very fast interplanetary shock proved to be an exceptionally efficient accelerator of particles. Because the shock weakens as it propagates away from the Sun, particles accelerated by diffusive shock acceleration leak out of the shock complex and stream along the interplanetary magnetic field toward the Earth. The particles experience some scattering between the shock and detectors on spacecraft at 1AU. As we see, energetic wave particles were collected and counted by spacecraft such as ACE, WIND and IMP8 at 1AU (e).

e. Image
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The MeV electron and proton fluxes as measured by the COSTEP instrument onboard SOHO. The first high energy electrons were detected at 10:38 UT. The long lasting intensity-time profiles are characteristic for fast interplanetary shocks associated with halo CMEs propagating towards Earth. Time resolution of the measurements shown is one minute. Note the particle intensity decreases on July 15/16 when the CME passed over SOHO. Courtesy of SOHO/COSTEP consortium. SOHO is a project of international cooperation between ESA and NASA.

The size and energy of the Bastille Day shock let to the acceleration of so many energetic particles that the particle detectors were completely saturated and became deactivated (f-k).

f. Image
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The ACE spacecraft monitors the solar wind at the Earth-Sun L-1 point, upstream from Earth. It can warn us of oncoming solar wind shocks about an hour before they collide with Earth's magnetosphere, so we will have advance warning of geomagnetic storms. The Bastille Day shock caused the acceleration of so many energetic particles that ACE's solar wind plasma monitors were disabled. However, the magnetometers continued to function.

g. Image
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h. Image
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i. Image
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Each of the three panels shows the (left) density (cm-3), (middle) flow speed (km/s), (right) temperature (K), and (bottom) magnetic field strength (nT) measured by the Ulysses SWOOPS and Magnetic Field instruments for the three shocks observed between days 203 - 214 of 2000 (21 July, 24 July, 1 August 2001). The shocks are indicated by vertical dashed lines.

j. Image
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k. Image
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A full halo coronal mass ejection recorded on July 14, 2000, by SOHO's C2 (left) and C3 (right) coronagraphs. "Halo events" are CMEs aimed toward the Earth. As they loom larger and larger they appear to envelop the Sun, forming a halo around our star. The many speckles in the latter half of this animation are energetic particles from a related solar flare bombarding SOHO's electronic detectors. Courtesy of SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.

The Bastille Day shock complex arrived shortly after the energetic particle detectors were swamped. However, the remaining spacecraft instruments observed the very intricate complex and discovered as many as five distinct shocks (l).

l. Image
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Initial (a) density, (b) flow velocity, (c) temperature, and (d) magnetic field conditions used for the Bastille Day simulation by Zank et al 2001 and Wang and Richardson 2001. Between Day 190 and Day 205 ACE SWEPAM and MAG 5 minute averaged and 30 minute averaged (where there are gaps in the 5 minute averages) data are used as input to the numerical simulation. From Day 205 onward the values at 1 AU return to those used to generate the initial steady state solution.

In the simulations we see the five shocks merging within 5-10 AU to form a single forward-reverse shock pair. This is the typical manner in which a GMIR is formed. The large global region of enhanced solar wind density and magnetic field acts as an outwardly traveling barrier to incoming cosmic rays. We discover that neutron monitors, which effectively measure cosmic ray intensity on the surface of the Earth, saw a decrease in the cosmic ray intensity after the passage of the Bastille Day shock. The decrease was followed eventually by a gradual recovery of the cosmic ray intensity as cosmic rays began to leak around and through the GMIR (m).

m. Image
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Comparison of Forbush decreases observed around the time of the Bastille Day (2000) activity with prior decreases associated with 2-3 kHz emissions in the outer heliosphere. Light lines show relative hourly count rates recorded by neutron monitors in Thule, Greenland and McMurdo, Antarctica. The heavy line is their average. Each monitor was normalized to an index value of 100 on the first day plotted. These data are available at .

On January 12, 2001, the now-merged Bastille Day shock was observed at Voyager 2 at 62 AU (n-o).

n. Image
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o. Image
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Left: Curves show intensities at Voyager 2 of protons with energies of about 1 (red) and 10 (blue) million electron volts (MeV). A 10 MeV proton has a speed of 44,000 km/s.
Right: This animation shows the propagation of the Bastille Day CME shock from 1 AU to Voyager 2 at 63 AU.

What will happen to the Bastille Day shock when it collides with the boundaries of the heliosphere? Will it light up the outer heliosphere with low frequency radio emissions when it collides with the heliopause?

Finally, did the Bastille Day shock accelerate pickup ions preferentially in the outer heliosphere, so that the pickup ions were then sufficiently energetic to be shock accelerated at the termination shock and thus become anomalous cosmic rays?

p. Image
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Schematic of the life cycle of a neutral atom. Interstellar neutral atoms stream into the supersonic solar wind, where some experience charge exchange to produce fast, outwardly streaming neutrals and pickup ions. The pickup ions are convected to the shock, where some are energized at the termination shock and become anomalous cosmic rays.

The Bastille Day event is a good example of how events on the surface of the Sun can affect the entire heliosphere and couple ultimately with the interstellar medium. Many similar examples reveal the interaction of heliospheric phenomena and the Galaxy.

q. Image
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A full halo coronal mass ejection recorded on July 14, 2000, by SOHO's C2 coronagraph. "Halo events" are CMEs aimed toward the Earth. As they loom larger and larger they appear to envelop the Sun, forming a halo around our star. The many speckles in the latter half of this animation are energetic particles from a related solar flare bombarding SOHO's electronic detectors. Courtesy of SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.

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