Solar Cycle 24

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Transport of magnetic flux is shown by observations.  In EUV, we see two sets of magnetic structures migrating.  The polar field reversed in about 2000.  The timescale of reversal between north and south are not the same.  A more careful look at the magnetic flux shows the northern hemisphere reversing sooner as well.  A comparison between MDI and WSO data shows a scale factor difference but otherwise good agreement.  One particular difference, MDI shows a large south pole field.  At higher latitudes (75 degrees) then there is a better correspondence with Ulysses.  The discrepancy in the timing of reversal is less at high latitudes.  Following small magnetic elements shows that meriodional flow is very small.  At low latitudes, small magnetic elements shows equator-ward motion.  Conclusions are that zonal or axisymmetrical structure of the solar cycle reveals transport of the magnetic flux between mid to high latitudes.  Migration of the zonal neutral line defines the reversal of the magnetic field during the solar cycle.  The transport of the magnetic energy is a complex process.
Transport of magnetic flux is shown by observations.  In EUV, we see two sets of magnetic structures migrating.  The polar field reversed in about 2000.  The timescale of reversal between north and south are not the same.  A more careful look at the magnetic flux shows the northern hemisphere reversing sooner as well.  A comparison between MDI and WSO data shows a scale factor difference but otherwise good agreement.  One particular difference, MDI shows a large south pole field.  At higher latitudes (75 degrees) then there is a better correspondence with Ulysses.  The discrepancy in the timing of reversal is less at high latitudes.  Following small magnetic elements shows that meriodional flow is very small.  At low latitudes, small magnetic elements shows equator-ward motion.  Conclusions are that zonal or axisymmetrical structure of the solar cycle reveals transport of the magnetic flux between mid to high latitudes.  Migration of the zonal neutral line defines the reversal of the magnetic field during the solar cycle.  The transport of the magnetic energy is a complex process.
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=== Magnetic connectivity and coronal dynamics by H. Peter (Kiepenheuer-Institut) ===
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Studying loops on the Sun have used 1D models which are not very realistic or full 3D models but with limited resolution.  The idea is to braid magnetic field lines.  Currents are then generated and lead to ohmic dissipation and heating.  The code used is a 3D MDH model (so-called Pencil code).  A reasonable agreement of between model doppler shifts and observed shifts.  Same can be said of the emission measure.  1D models are not very good at this.  Looking at individual loops from this model show a spiky structure in the heating rate (maybe associated with "nanoflares").  Temperature profiles show wide variety.  The heating scale height is roughly exponential with a scale height of 10 Mm (heating is concentrated in the chromosphere).  Loops can also be seen to jump around which is further reason that 1D models which track a fixed loop are not reasonable.  Model suggests that small loops seen in the quiet Sun are really just a projection effect and not real.  The isothermal structure is much more complicated than just "traditional" loops.
== Working Groups ==
== Working Groups ==

Revision as of 20:39, 8 December 2008

Meeting
Name: Solar Cycle 24
Start Date: 07 Dec 2008
End Date: 12 Dec 2008
Location: Napa, CA, U.S.A.
Official Website: http://sprg.ssl.berkeley.edu/RHESSI/napa2008

Contents

Summary

The main goals of this meeting is to assess our current knowledge of solar activity to prepare for observations of the new major activity expected from the beginning of Carrington Cycle 24, and to encourage effective observations of this activity. There will be special emphasis on coordinating ACE, Hinode, STEREO, TRACE/SDO, SOHO, RHESSI and Wind observations, among others including ground-based. The conference is composed of a mixture of invited and contributed talks in plenary sessions, plus focused working groups and took place December 8 - December 12, 2008. The week before the AGU Fall meeting in San Francisco. The location of the meeting is The Embassy Suites Hotel in Napa, California (wine country north of San Francisco). Check-in 16:00, Check-out 12:00. For those staying at the hotel there is daily complimentary cooked-to-order breakfast and evening manager's reception. The registration fee itself covers the meeting, coffee breaks and the conference dinner. The official website can be found here

Notes

Solar Activity Cycles - Past and Present by D. Hathaway (Marshall Space Flight Center)

Will there be a cycle 24? New cycle active regions have begun to show up the last two months. The number of spotless days is more than twice that of the last few cycles. Also Cycle 23 is the longest cycle than the last few cycles. Looking at a stastics of older cycles turns out that cycle 23 is not out of the norm. On average the maximum occurs 4 years after the minimum. Statistics of cycles suggest that Cycle 24 may be small though cycles have been growing in sunspot number since 1700. Should we be worried about another Maunder Minimum (one of the grand minimum)? Grand minimum occur once every 400 years, we are currently in a grand maximum. "Prediction is very difficult especially about the future" (Niels Bohr). Forecasting an ongoing cycle is easiest about 2-3 into a new cycle (at the inflection point of the rise). Geomagnetic precursors suggest a (slightly) smaller than average cycle 24 or a much larger cycle depending on who you talk to. Geomagnetic indices (and polar field strength) are best at predicting solar cycles. Polar fields are currently weaker by a factor of 2 than previous solar minimum. Polar field indices predict a much smaller cycle 24. The first dynamo predictions suggest a cycle 24 will be much larger than cycle 23 though big cycles usually start early which has not been observed (some caveats suggest that independent confirmation of this prediction is necessary). Another dynamo model (Choudhuri 2007) predicts a small cycle 24 in keeping with geodynamic predictions (caveats also exist for this method).

Cycle 24 may be large or small! And should exist (we hope). We should know by the end of 2010.

A link to the powerpoint presentation should be placed here.

Recalibration of the Sunspot number and consequences for prediction of future activity and reconstructions of past solar behavior by L. Svalgaard (Stanford U.)

There are two real long-term sunspot numbers (group number and international number aka Wolf number). The two indices diverge 1875 making predictions difficult. People in the prediction business choose the index that works the best for their method.

The ratio of the two indices over the time of divergence show systematic changes based on changes in the human observers (~17% changes). Wolf's own sunspot (Wolf) numbers used past observations and show strange adjustements. The variation in declination of the geo north varies as a function of day and depends on the solar cycle. Wolf changed his sunspot numbers based on those measurements. Using this geomagnetic index it is possible to reconcile the difference between the two indices.

Data from the 19th century is currently in the process of being digitized for public distribution. The difference in the two indices is a problem which needs to be resolved for predictions to go forward.

A link to the powerpoint presentation should be placed here.


Evolving views of solar flares, and targets for Cycle 24 by L. Fletcher (U. of Glasgow)

Looking back at cycle 18, Giovanelli (1948) had a very nice model of a solar flares. Concluded that most of the radiation of flares comes from the chromosphere (as opposed to the corona). Energy storange is in the corona. But no knowledge of nonthermal particles and CMEs.

What have we learned since then? Coronal energy is released by topological changes to coronal magnetic fields (reconnection). Up to 50% of magnetic energy released goes to nonthermal particles. Accelerated particles are present in both corona and chromosphere. The chromosphere is heated and radiates stronly and expands.

Topological changes in magnetic field create currents which are hard to work out but seems to be located near the neutral line. Halpha flare ribbons and serperatrix layers are well corolated. But how do currents change during a flare?

Energy transport in flares is mostly done by nonthermal particles. 1028 to 1029 erg per s goes into electrons. Similar energy in ions. The Accelerator is, of course, still unknown (stochastic? DC fields?).

Based on chromospheric HXRs beam models >10% of coronal electrons are accelerated and leave the corona each second. This is very large (usually refered to as the Numbers problem). But recent angular distribution measurement suggests that beams do not really exist which mean that we are using the wrong model.

Changes in the coronal magnetic structure may show up in Need to keep our eye on the chromosphere since most of the energy is released there? Can the current close in the chromosphere?

Main messages, flares are magnetic need to consider energy transport in light of this. The corona is a plasma, we must consider plasma effects such as waves and return currents etc. Most of the flare energy is made manifest in the chromosphere.

A link to the powerpoint presentation should be placed here.


Global Magnetic Field Evolution by A. Van Ballegooijen

Can deduce global fields by extrapolating photospheric fields (potential, nonpotential, MHD, force-free, PFSS). Observations suggest that fields are definitely not potential. Active regions are thought to be loops which emerge from deep twisted magnetic flux tubes. In the corona the field spreads out. Fan (2008) has simulated the rise of these flux tubes. This model suggests that active regions should be much larger (azimuthally). Something is therefore missing from this model. Convections may be the key. Fan (2003) simulated with convective flows. Speakers own work concentrated on the coupling between the corona and convection zone. Magnetic diffusion in convection zone is modified to conserve magnetic helicity. This leads to a build up of magnetic helicity in the corona which eventually leads to a loss of equilibrium. They also modeled how two active regions interact with each other. Field lines connect between active regions. The reserval of the Sun's polar magnetic field can be explained by solar differential rotation. The effects of active regions after they have decayed and submerged are not well understood. A new 2D model by speaker (in prep) seeks to understand this. Active regions tilt the underlying horizontal field consistent with Joy's law. The cumulative effects of these titls ends up reverses the poloidal field. Four different processes may be involved in the dissipation of the Sun's toroidal field.

Some conclusions, the upper convection zone is important has it stops the submergence of sheared magnetic fields. Active regions need to shed helicity by ejecting it (i.e. CMEs). Active regions effect the underlying submerged fields.

Relationship between the High and Mid latitude Solar Magnetic Field by E. Benevolenskaya given by T. Hoeksema (Stanford U.)

Transport of magnetic flux is shown by observations. In EUV, we see two sets of magnetic structures migrating. The polar field reversed in about 2000. The timescale of reversal between north and south are not the same. A more careful look at the magnetic flux shows the northern hemisphere reversing sooner as well. A comparison between MDI and WSO data shows a scale factor difference but otherwise good agreement. One particular difference, MDI shows a large south pole field. At higher latitudes (75 degrees) then there is a better correspondence with Ulysses. The discrepancy in the timing of reversal is less at high latitudes. Following small magnetic elements shows that meriodional flow is very small. At low latitudes, small magnetic elements shows equator-ward motion. Conclusions are that zonal or axisymmetrical structure of the solar cycle reveals transport of the magnetic flux between mid to high latitudes. Migration of the zonal neutral line defines the reversal of the magnetic field during the solar cycle. The transport of the magnetic energy is a complex process.


Magnetic connectivity and coronal dynamics by H. Peter (Kiepenheuer-Institut)

Studying loops on the Sun have used 1D models which are not very realistic or full 3D models but with limited resolution. The idea is to braid magnetic field lines. Currents are then generated and lead to ohmic dissipation and heating. The code used is a 3D MDH model (so-called Pencil code). A reasonable agreement of between model doppler shifts and observed shifts. Same can be said of the emission measure. 1D models are not very good at this. Looking at individual loops from this model show a spiky structure in the heating rate (maybe associated with "nanoflares"). Temperature profiles show wide variety. The heating scale height is roughly exponential with a scale height of 10 Mm (heating is concentrated in the chromosphere). Loops can also be seen to jump around which is further reason that 1D models which track a fixed loop are not reasonable. Model suggests that small loops seen in the quiet Sun are really just a projection effect and not real. The isothermal structure is much more complicated than just "traditional" loops.

Working Groups

Group B (Fast Solar Wind & Plumes)

A summary of the work of this group can be found here.

Group C (Magnetic Field Evolution)

A summary of the work of this group can be found here.

Group D (Global event energetics)

A summary of the work of this group can be found here.

Group E (Flares)

A summary of the work of this group can be found here.

Group F (CMEs)

A summary of the work of this group can be found here.

Group G (Microflares and Nanoflares)

A summary of the work of this group can be found here.

Group H (The Chromosphere)

A summary of the work of this group can be found here.

Group I (Active Region Loops)

A summary of the work of this group can be found here.

Group J (Filaments and Prominences)

A summary of the work of this group can be found here.

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