Equatorial Magnetohydrodynamic Shallow Water Waves in the Solar Tachocline
Equatorial Magnetohydrodynamic Shallow Water Waves in the Solar Tachocline
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DOI：
10.3847/1538-4357/aab26f
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发表时间：
2018-02
期刊：
影响因子：
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通讯作者：
T. V. Zaqarashvili
中科院分区：
文献类型：
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作者：
T. V. Zaqarashvili
研究了环形磁场对太阳跃层浅水波动力学的影响。塔乔跃层上部超跃层的亚绝热温度梯度导致地表重力速度显著降低，从而导致赤道附近的波被捕获，Rossby波周期增加到太阳周期的时间尺度。得到了上层跃层条件下所有赤道磁流体浅水波的频散关系，并进行了解析和数值求解。结果表明，环向磁场将赤道Rossby波和Rossby重力波分裂为快模和慢模。对于一个合理的重力折减值，全球赤道快速磁Rossby波(以赤道范围为空间尺度)具有11年的周期，与活动周期的时间尺度相匹配。解决方案局限在赤道之间的纬度±20°-40°，与太阳黑子活动带重合。赤道慢磁Rossby波具有90-100年的周期，类似于观测到的周期强度的长期调制，即Gleissberg循环。赤道磁开尔文波和慢磁Rossby重力波的周期为1-2年，可能与观测到的年振荡和准两年振荡相对应。赤道快速磁Rossby重力波和磁惯性重力波具有数百天的周期，可能是观测到的Rieger型周期的原因。因此，上超跃层中的赤道MHD浅水波可以捕捉到观测到的太阳活动变化的所有时间尺度，但有必要进行详细的分析和数值研究，以得出这些波与太阳发电机之间的联系的确切结论。
The influence of a toroidal magnetic field on the dynamics of shallow water waves in the solar tachocline is studied. A sub-adiabatic temperature gradient in the upper overshoot layer of the tachocline causes significant reduction of surface gravity speed, which leads to trapping of the waves near the equator and to an increase of the Rossby wave period up to the timescale of solar cycles. Dispersion relations of all equatorial magnetohydrodynamic (MHD) shallow water waves are obtained in the upper tachocline conditions and solved analytically and numerically. It is found that the toroidal magnetic field splits equatorial Rossby and Rossby-gravity waves into fast and slow modes. For a reasonable value of reduced gravity, global equatorial fast magneto-Rossby waves (with the spatial scale of equatorial extent) have a periodicity of 11 years, matching the timescale of activity cycles. The solutions are confined around the equator between latitudes ±20°–40°, coinciding with sunspot activity belts. Equatorial slow magneto-Rossby waves have a periodicity of 90–100 yr, resembling the observed long-term modulation of cycle strength, i.e., the Gleissberg cycle. Equatorial magneto-Kelvin and slow magneto-Rossby-gravity waves have the periodicity of 1–2 years and may correspond to observed annual and quasi-biennial oscillations. Equatorial fast magneto-Rossby-gravity and magneto-inertia-gravity waves have periods of hundreds of days and might be responsible for observed Rieger-type periodicity. Consequently, the equatorial MHD shallow water waves in the upper overshoot tachocline may capture all timescales of observed variations in solar activity, but detailed analytical and numerical studies are necessary to make a firm conclusion toward the connection of the waves to the solar dynamo.