Space Weather Center
Tel Aviv University
What is Space Weather and why is it important to monitor it?
Space Weather
Space weather refers to environmental conditions in interplanetary space that change under the influence of the Sun over a period of time from minutes to several days. Longer exposure to the Sun is called space climate. Space weather determines the distribution of solar wind currents and streams, as well as energetic particles with energies up to MeV and even more energetic cosmic rays of both solar and galactic origin. It controls the interaction between the solar wind and the Earth's magnetic field and causes changes in the Earth's magnetosphere, ionosphere and atmosphere. Space weather monitoring is critical to reducing risks and protecting ground infrastructure and space missions. The risks that can affect Earth include energetic particles from solar flares, interplanetary shocks and other combined sources of accelerated protons and electrons. These particles can disrupt satellites, communication systems, and pose radiation hazards to astronauts. X-rays and UV radiation from solar flares can affect the Earth's atmosphere, impacting radio communication. Intense geomagnetic storms can induce electric currents in power grids, leading to transformer failures and widespread power outages. Monitoring space weather is essential to mitigate these risks and protect our technology-dependent infrastructure.
Coronal Holes
Coronal holes (CHs) are areas in the solar outer atmosphere which appear as large dark spots in the extreme ultraviolet (EUV) and X-ray wavelength imagery. Coronal holes are characterized by lower-density and cooler plasma compared to their surroundings in the solar corona. The lower temperature in coronal holes that make them dark in images is attributed to the open magnetic field lines that facilitate the escape of solar wind. With less magnetic confinement, the plasma in coronal holes is not as strongly heated as in other regions.
The solar wind accelerates in coronal holes, leading to formation of high-speed flows in the heliosphere. The fast solar wind in coronal holes is a result of the unobstructed flow of charged particles along open magnetic field lines. Without the constraining effects of closed magnetic loops, solar wind particles can stream away more freely, resulting in higher velocities. Coronal holes move from high latitudes in the minimum of solar activity to the equator in the solar activity maximum, as shown in the videos on the right.
When a coronal hole stretches into the solar wind, it creates a flow that twists along the Parker spiral because of the rotation of its solar source and simultaneously expands in all directions. As a result of its interaction with the surrounding slower solar wind, a region called Stream Interaction Region (SIR) forms as a shell at the edges of the high speed flow from coronal holes.
SIRs are recognized in the in situ spacecarft measurements of the solar wind parameters by an arrival of a compressed region with the enhanced density and the interplanetary magnetic field in the background of ordinary speed values at their leading edge. Then a high-speed flow follows. Long-lived SIRs are called Corotating Interaction Regions (CIRs). SIRs/CIRs and high-speed flows from coronal holes can impact the terrestrial magnetosphere and cause geomagnetic storms and increased auroral activity.
minimum of solar activity
maximum of solar activity
Halo Coronal Mass Ejection
How a Coronal Mass Ejection is born
Coronal Mass Ejections
Coronal Mass Ejections (CMEs) are massive explosions in the solar corona, bringing (10^14–10^16) gr of magnetized coronal plasma into the interplanetary space (see an example of CME observation with coronographs in the upper video). The energy of a typical CME can range from about 1 x 10^22 to 1 x 10^25 joules, which is equivalent to millions of 100-megaton hydrogen bombs. CMEs are often related to solar flares and occur as a result of magnetic reconnection.
Before solar eruptions, there is a period of stress in the magnetic field that lasts for a few days to weeks. During this time, the magnetic energy and twists in the solar atmosphere build up. In simple words, these eruptions happen when something goes wrong in structures with a lot of magnetic energy (see the video in the middle).
Imagine it like a rubber band getting stretched too much. Two forces are at play here: one that pushes things outward and another that tries to keep things in place. When the rubber band (magnetic confinement) cannot hold anymore, we get a solar eruption. This can happen in two ways: either it is a bit like a rubber band snapping back (magnetic reconnection) or there is an explosion because of some instability in the system. After this happens, sometimes the magnetic loops in the solar atmosphere collapse back into a new stable shape, which resemble the rubber band settle down after all the excitement.
CMEs can explode with a broad angular range from the smallest conceivable angular width to a complete circumferential extent of 360 degrees around the Sun. A type of CMEs with a vast angular width is called Halo (showed in the upper video). Halo CMEs has 4 types: I, II, III, and IV. The numbers reflect the angular width of a particular CME. It is II if the angular width> 90 deg, III if the angular width >180 deg, and IV if the angular width >270 deg.
The number of CMEs follows the solar cycle, i.e. it grows in the solar maximum and decays in the solar minimum. In the solar wind, CMEs propagate faster than the surrounding plasma and are called high-speed Interplanetary Coronal Mass Ejections (ICMEs). ICMEs directed towards the Earth can cause intense geomagnetic storms.​
What Coronal Mass Ejections are and what they can do to the Earth
Geomagnetic Storms
Geomagnetic storms, the severe disturbances of the terrestiral magnetic field, are major consequenses of variable space weather. The terrestrial magentosphere exists, first, the Earth has its own magnetic field, and second, because there is the solar wind flowing from the Sun and impacting the terrestrial magnetic field. Picture the Earth surrounded by its own magnetic field, like an invisible shield. The Sun constantly sends out the solar wind, shaping this magnetic shield. Instead of being round, it has a tail called the magnetotail, where most of the magnetospheric energy is stored.
The border between this shield and space plasma is kept in check by the solar wind pressure from one side and the Earth's magnetic field pressure from the other. Actually, two borders are formed – the bow shock (formed similar to the way as a shock is formed by a plane flying with a supersonic speed), and the magnetopause. The magnetopause separates a turbulent region, called the magnetosheath, downstream of the shock from the pure magnetic field of the Earth.
Althought the Earth's magnetic shield responds to any changes in the solar wind, geomagnetic storms are triggered by a unique state of the solar wind that leads to magnetic reconnection at the sub-solar side of the magnetopause and subsequent reconnection in the night-side magnetotail, releasing stored energy. This scenario is known to align with an increase in the solar wind electric field, known as the "VBz paradigm." To stimulate a geomagnetic storm, the vertical component of the interplanetary magnetic field (Bz) in the Geocentric Solar Ecliptic coordinate system should be negative, and the solar wind speed V must be increased to bring more energy to the magnetosphere. These conditions of the elevated speed and the negative (southward) Bz often occur when CMEs and SIRs discussed above reach the Earth.
Contact Us
If you have any questions or collaboration suggestions, please contact Dr. Olga Khabarova
olgakhabar(at)tauex.tau.ac.il
fax: +972-3-6409282
Department of Geophysics, Faculty of Exact Sciences, Kaplun building.
Tel-Aviv University , P.O.B 39040, Ramat Aviv, 6139001, Tel Aviv 69978