March 31, 2001 geomagnetic storm
by Volker Grassmann, DF5AI, May 27, 2005
In this section, scientific background information will provide a brief overview on the world-wide effects associated with the March 31, 2001 geomagnetic storm. The material focuses on selected aspects and addresses radio amateurs rather than experts in solar and geophysical reasearch. All information is freely available in the internet, i.e. the reader may access the original sources and additional background information by a simple mouse click.
Solar wind shock front travelling towards the Earth
On March 23, 2001, the sun spot region 9393 was first identified at the eastern limb of the Sun. While this region moved towards the disk center in the following days (see fig. 1), a number of solar flares were detected including a flare rated X-class, i.e. the most potent designation. An eruption, called a coronal mass ejection (CME) near the active region 9393 hurled a high-speed solar wind stream of electrified, magnetic gas towards the Earth on March 28, 2001 , . Typically, solar wind shock fronts travel at a speed of a few hundred kilometers per second, i.e. the solar wind travels the distance from the Sun to the Earth (150 million kilometers) in two to three days.
However, before reaching the Earth, the shock fronts passes the human mankind's sentry in space, i.e. the ACE spacecraft. ACE orbits the so-called L1 Lagrange point, i.e. the point of Earth-Sun gravitational equilibrium about 1.5 million km from Earth and 148.5 million km from the Sun. On March 31, 2001 at 0023 UT, the large high-speed solar wind shock front arrived at the ACE spacecraft, see fig. 2. At 0053 UT, the shock front pushed into the dayside magnetosphere causing geomagnetic field disturbance which was observed around the world , . The intense compression of the Earth's magnetosphere temporarily left geostationary satellites outside of the Earth's magnetic field , , i.e. the magnetosphere's so-called bow shock crossed the orbit of geostationary satellites towards Earth.
Effects on Earth
On Earth, we would consider a density of 150 to 200 particles per cubic centimeter a perfect vacuum, more or less. However, the same amount of charged particles composed the leading edge of the solar wind stream  which in fact represents a large solar wind shock front. The expanding solar wind drags also the solar magnetic field outward, forming the so-called interplanetary magnetic field (IMF). Its field lines are said to be "frozen in" to the solar wind plasma because the field lines cannot contact back to the Sun. At the magnetopause, the Earth's magnetic field and the IMF come into contact. If the IMF points south ("southward Bz"), then the IMF can partially cancel the Earth's magnetic field at the point of contact, i.e. the two fields link up. In this situation, a field line from Earth connects directly into the solar wind, i.e. energy may be injected into the Earth's magnetosphere which may cause widespread Auroras and other type of phenomena .
At 0023 UT the ACE spacecraft measured a southward IMF for a short period of time (see fig. 3). The initial geomagnetic field disturbance was therefore a short-duration event . The main storm phase commenced at 02 UT when the northward IMF turned south and this situation persisted until 08 UT, neglecting short periods of time in which the IMF returned to north (fig. 3). From 02-08 UT, sky watchers spotted magnificent Northern Lights even in Texas and Mexico, see e.g. . In North America, the magnetic field disturbance was particularly intensive along the US-Canada border reaching 200-600 nT/min . In New York, some power system disturbances were observed  due to geomagnetic induced currents (GIC): geomagnetic variations produce electric fields which induce currents in power lines and oil and gas pipelines appearing as a slowly-varying DC current .
In the southern hemisphere, Aurora Australis (Southern Lights) was spotted e.g. in New Zealand at 07-14 UT (see e.g. ) which corresponds to the phase of northward Bz. At 14 UT, the polarity of the IMF turned sharply southward recovering the storm activity. The southward Bz remained for the rest of the day in combination with the arrival of a second solar wind shock front at 22 UT, see fig. 2. In Europe, substantial geomagnetic field disturbances were found over the entire time interval and particularly large impulses were observed from Sweden through the United Kingdom , see fig. 4.
Large pertubations occured in the upper atmosphere and in the ionosphere. In the F-region (300 to 400 km), the MIT incoherent scatter radar detected electrical field strengths of 100 mV/m which represent the largest ever recorded electric field at the Haystack Observatory, U.S.A. . In the E-region (100 km), the neutral wind velocities were 500 m/s resulting from heat deposition by the Aurora (during quit solar times, the neutral wind speed is about 50 to 100 m/s in mid-latitudes) . A plume of storm enhanced electron density (SED) was found in a geographical band from New England to the Great Lakes into Canada in which ion velocities of about 1000 m/s were detected . In Europe, ionosonde measurements at Chilton, United Kingdom, revealed a very strong negative phase of more than -150 percent in f0F2 , see fig. 5. Pertubations in the total electron content of the ionosphere (TEC) between Tromsö, Norway, and Ankara, Turkey indicated a highly disturbed ionosphere in time and space over Europe . Finally, radio amateurs in Europe, North America and even Australia reported various phases of strong Auroral backscatter between 28 and 432 MHz which will be analysed in more detail in this project.
See also the discussion of the Auroral activity and the hemispheric input power in a separate section of this web site.