A SOLAR FLARE is defined as a sudden, rapid, and intense variation in brightness.

1. A SOLAR FLAREis defined as a sudden, rapid, and intense variation in brightness. • A solar flare occurs when magnetic energy that has been built up in the solar atmosphere is suddenly released. • Radiation is emitted across the spectrum -- radio, visible, x-ray, gamma-rays. • The amount of energy released is equivalent to millions of 100-megaton hydrogen bombs exploding at the same time. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

2. Energy is released when intense magnetic fields relax back to a quiescent state (much like a stretched rubber band). The energy is transmitted to the particles trapped on the filed lines. The electrons are thought to be accelerated by the collapse of stretched magnetic field lines high above the solar surface (``magnetic reconnection''). The accelerated electrons heat up the thermal plasma in the loop directly, and indirectly by “chromospheric evaporation”. The soft (thermal) x-rays seen by TRACE reflect this heating. The hard X-rays from the base of the active region are ``bremsstrahlung'', or ``braking radiation'', caused by electrons slamming into the dense gases at the bottom of the corona. This heated chromospheric gas rises up (“chromospheric evaporation”) and also heats the thermal plasma in the loop. In solar flares electrons are both heated to high temperatures, and accelerated ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

3. Bremsstrahlung Radiation High-energy electrons are decelerated through attraction by positively-charged “low-energy” ions. When electrons are decelerated, they give off radiation called “bremsstrahlung” (or “braking”) radiation, usually in the form of “hard” x-rays, i.e., energies of order 10-100 keV The type of radiation given off by the heated “thermal” (10-30 million K) plasma is different, consisting of “soft” x-rays (typically 1-10 keV). Some thermal bremsstrahlung from very hot thermal plasma (> 30 million K) is also emitted ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

4. There are typically three stages to a solar flare (each lasting from ~seconds to ~1 hour). precursorstage: release of magnetic energy is triggered. Soft x-ray emissions. impulsive stage: protons and electrons are accelerated to energies exceeding 1 Mev; radio waves, hard x-rays, and gamma rays are emitted. decaystage: gradual build up and decay of soft x-rays. Observations of hard x-rays (10-100 keV) allow us to study the accelerated electrons and the hottest plasma in flares Observations of soft x-rays (1-10 keV) allow us to study the thermal plasma component ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

6. The relationship between the nonthermal (accelerated) electrons and the hottest thermal electrons can be studied by observing the time evolution of both components during a flare. Likewise, the relationship between these energetic components and somewhat cooler thermal plasma can be studied by comparing the hard x-ray observations with the evolution of the soft x-ray emission. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

7. Optical Classification of Flares The optical (as seen in Hydrogen-alpha light) classification of a flare is made using a two-character designation. For example, a 1B designation indicates a ``brilliant” intensity flare covering a corrected area between 100 and 249 millionths of the solar hemisphere. FLARE BRIGHTNESS CATEGORIES: F: FAINT N: NORMAL B: BRILLIANT ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

8. Frequency of Optical Solar Flares During Cycles 20-21 ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

9. The left categories are broken down into nine subcategories based on the first digit of the actual peak flux. For example, a peak flux of 5.7 x 10-2 ergs/cm2-sec is reported as a M5 soft x-ray flare. ClassificationX-Ray Flux (ergs/cm2-sec) C 10-3 M 10-2 X 10-1 X-Ray Classification of Flares The most common x-ray index is based on the peak energy flux of the flare in the 1 to 8 Å x-ray band measured by geosynchronous satellites. These measurements must be made from space, since the Earth’s atmosphere absorbs all solar x-rays before they reach the Earth’s surface. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

10. Bastille Day Event July 14, 2000 ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

11. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

12. CORONAL MASS EJECTIONS (CMEs) ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

13. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

14. CME Rate Solwind (1979-1984) SMM (1984-1989) SOHO (1996-2002) 27d Average 2800MHz Solar Flux ----- (Max=254) ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

15. SOHO LASCO CME Latitude Distributions 1996 2000 ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

16. There is good association between CMEs and Long-Duration-Event (LDE) soft X-ray flares ( or gradual flares). How are Flares and CME's Related? • The need to release built-up magnetic field energy leads to both flares and CMEs. • CMEs involve the eruption of a confining magnetic field, but only a special class of flares called Solar Proton Events (SPEs) involve the breaking apart of magnetic field lines and spewing out of highly energetic protons. Sometimes flares are categorized as impulsive (more compact; last minutes; occurring low in the corona) and gradual. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

17. The Bastille-day flare was ‘X-class’ and accompanied by one of the largest solar energetic proton events ever recorded c3714 ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

18. Solar Physics in the Media The great solar storms of October/November 2003 ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

19. Sun’s Radiation and the Upper Atmosphere

20. 81-day Mean Density Normalized to 390 km Derived from Precise Orbit Determination of MGS (370 x 437 km orbit; perigee near -40º to -60º latitude, 1400 LT) 81-day mean F10.7 solar flux at 1 AU 81-day mean F10.7 solar flux at Mars 81-day mean density Note: Each density determination is made over 3-5 Mars days, and is a longitude average, so there is no possibility to derive longitude variability, e.g., as seen in MGS accelerometer data. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

21. Fit for density (10-18 cm-3): Least-Squares Fit to Exosphere Temperature Derived from Observed Densities and DTM-Mars (Lemoine and Bruinsma, 2002) zonal mean dust optical depth ±30o latitude avg. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

22. Earth-Mars Comparative Response to Long-Term Changes in Solar flux Mars - MGS Ls variation removed Earth - msise90 40o latitude Mars is ~36% - 50% as responsive to solar flux received at the planet, compared to Earth, consistent with Forbes et al. (2006) ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere